diff --git a/Makefile b/Makefile
index 27bdf2f..87d5286 100644
--- a/Makefile
+++ b/Makefile
@@ -35,3 +35,17 @@ ifc-dev:
 ifc-test:
 	cd ifc && make test
 
+kifu-core/dev:
+	cd kifu/kifu-core && make test
+
+kifu-core/test:
+	cd kifu/kifu-core && make test
+
+kifu-core/test-oneshot:
+	cd kifu/kifu-core && make test-oneshot
+
+kifu-gtk:
+	cd kifu/kifu-gtk && make release
+
+kifu-gtk/dev:
+	cd kifu/kifu-gtk && make dev
diff --git a/flake.nix b/flake.nix
index d44619f..e223b90 100644
--- a/flake.nix
+++ b/flake.nix
@@ -38,6 +38,7 @@
             pkgs.pkg-config
             pkgs.sqlite
             pkgs.rustup
+            pkgs.cargo-nextest
             pkgs.crate2nix
           ];
           LIBCLANG_PATH="${pkgs.llvmPackages.libclang.lib}/lib";
diff --git a/kifu/kifu-core/Cargo.lock b/kifu/kifu-core/Cargo.lock
index 59b31e4..6880834 100644
--- a/kifu/kifu-core/Cargo.lock
+++ b/kifu/kifu-core/Cargo.lock
@@ -26,6 +26,15 @@ version = "1.0.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "baf1de4339761588bc0619e3cbc0120ee582ebb74b53b4efbf79117bd2da40fd"
 
+[[package]]
+name = "grid"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0634107a3a005070dd73e27e74ecb691a94e9e5ba7829f434db7fbf73a6b5c47"
+dependencies = [
+ "no-std-compat",
+]
+
 [[package]]
 name = "hermit-abi"
 version = "0.2.6"
@@ -39,6 +48,8 @@ dependencies = [
 name = "kifu-core"
 version = "0.1.0"
 dependencies = [
+ "grid",
+ "thiserror",
  "tokio",
 ]
 
@@ -85,6 +96,12 @@ dependencies = [
  "windows-sys",
 ]
 
+[[package]]
+name = "no-std-compat"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b93853da6d84c2e3c7d730d6473e8817692dd89be387eb01b94d7f108ecb5b8c"
+
 [[package]]
 name = "num_cpus"
 version = "1.15.0"
@@ -193,6 +210,37 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "syn"
+version = "2.0.12"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "79d9531f94112cfc3e4c8f5f02cb2b58f72c97b7efd85f70203cc6d8efda5927"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "unicode-ident",
+]
+
+[[package]]
+name = "thiserror"
+version = "1.0.40"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "978c9a314bd8dc99be594bc3c175faaa9794be04a5a5e153caba6915336cebac"
+dependencies = [
+ "thiserror-impl",
+]
+
+[[package]]
+name = "thiserror-impl"
+version = "1.0.40"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "f9456a42c5b0d803c8cd86e73dd7cc9edd429499f37a3550d286d5e86720569f"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn 2.0.12",
+]
+
 [[package]]
 name = "tokio"
 version = "1.26.0"
@@ -221,7 +269,7 @@ checksum = "d266c00fde287f55d3f1c3e96c500c362a2b8c695076ec180f27918820bc6df8"
 dependencies = [
  "proc-macro2",
  "quote",
- "syn",
+ "syn 1.0.109",
 ]
 
 [[package]]
diff --git a/kifu/kifu-core/Cargo.toml b/kifu/kifu-core/Cargo.toml
index ad2dec2..f7c8c30 100644
--- a/kifu/kifu-core/Cargo.toml
+++ b/kifu/kifu-core/Cargo.toml
@@ -6,4 +6,6 @@ edition = "2021"
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
 [dependencies]
-tokio = { version = "1.26", features = [ "full" ] }
+tokio       = { version = "1.26", features = [ "full" ] }
+grid        = { version = "0.9" }
+thiserror   = { version = "1" }
diff --git a/kifu/kifu-core/Makefile b/kifu/kifu-core/Makefile
new file mode 100644
index 0000000..bfcdb8e
--- /dev/null
+++ b/kifu/kifu-core/Makefile
@@ -0,0 +1,6 @@
+
+test:
+	cargo watch -x 'nextest run'
+
+test-oneshot:
+	cargo nextest run
diff --git a/kifu/kifu-core/src/api.rs b/kifu/kifu-core/src/api.rs
index 983a5bc..5abd9d4 100644
--- a/kifu/kifu-core/src/api.rs
+++ b/kifu/kifu-core/src/api.rs
@@ -10,8 +10,8 @@ pub enum Request {
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct PlayStoneRequest {
-    pub column: u8,
-    pub row: u8,
+    pub column: usize,
+    pub row: usize,
 }
 
 #[derive(Clone, Debug)]
diff --git a/kifu/kifu-core/src/board.rs b/kifu/kifu-core/src/board.rs
new file mode 100644
index 0000000..55a3fc5
--- /dev/null
+++ b/kifu/kifu-core/src/board.rs
@@ -0,0 +1,633 @@
+use crate::{BoardError, Color, Size};
+use std::collections::HashSet;
+
+#[derive(Clone, Debug)]
+pub struct Board {
+    pub size: Size,
+    pub groups: Vec<Group>,
+}
+
+impl std::fmt::Display for Board {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> Result<(), std::fmt::Error> {
+        write!(f, "   ")?;
+        // for c in 'A'..'U' {
+        for c in 0..19 {
+            write!(f, "{:2}", c)?;
+        }
+        writeln!(f, "")?;
+
+        for row in 0..self.size.height {
+            write!(f, " {:2}", row)?;
+            for column in 0..self.size.width {
+                match self.stone(&Coordinate { column, row }) {
+                    None => write!(f, " .")?,
+                    Some(Color::Black) => write!(f, " X")?,
+                    Some(Color::White) => write!(f, " O")?,
+                }
+            }
+            writeln!(f, "")?;
+        }
+        Ok(())
+    }
+}
+
+impl PartialEq for Board {
+    fn eq(&self, other: &Self) -> bool {
+        if self.size != other.size {
+            return false;
+        }
+
+        for group in self.groups.iter() {
+            if !other.groups.contains(&group) {
+                return false;
+            }
+        }
+        for group in other.groups.iter() {
+            if !self.groups.contains(&group) {
+                return false;
+            }
+        }
+        return true;
+    }
+}
+
+impl Board {
+    pub fn new() -> Self {
+        Self {
+            size: Size {
+                width: 19,
+                height: 19,
+            },
+            groups: Vec::new(),
+        }
+    }
+
+    pub fn from_coordinates(
+        mut coordinates: impl Iterator<Item = (Coordinate, Color)>,
+    ) -> Result<Self, BoardError> {
+        coordinates.try_fold(Self::new(), |board, (coordinate, color)| {
+            board.place_stone(coordinate, color)
+        })
+    }
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Hash, Eq)]
+pub struct Coordinate {
+    pub column: usize,
+    pub row: usize,
+}
+
+impl Board {
+    pub fn place_stone(mut self, coordinate: Coordinate, color: Color) -> Result<Self, BoardError> {
+        if let Some(_) = self.stone(&coordinate) {
+            return Err(BoardError::InvalidPosition);
+        }
+
+        let mut friendly_group = self
+            .adjacencies(&coordinate)
+            .into_iter()
+            .filter(|c| self.stone(c) == Some(color))
+            .filter_map(|c| self.group(&c).map(|g| g.coordinates.clone()))
+            .fold(HashSet::new(), |acc, set| {
+                acc.union(&set).cloned().collect()
+            });
+
+        friendly_group.insert(coordinate.clone());
+
+        self.groups
+            .retain(|g| g.coordinates.is_disjoint(&friendly_group));
+        let friendly_group = Group {
+            color,
+            coordinates: friendly_group,
+        };
+        self.groups.push(friendly_group.clone());
+
+        let adjacent_groups = self.adjacent_groups(&friendly_group);
+        for group in adjacent_groups {
+            if self.liberties(&group) == 0 {
+                self.remove_group(&group);
+            }
+        }
+
+        if self.liberties(&friendly_group) == 0 {
+            return Err(BoardError::SelfCapture);
+        }
+
+        Ok(self)
+    }
+
+    pub fn stone(&self, coordinate: &Coordinate) -> Option<Color> {
+        self.groups
+            .iter()
+            .find(|g| g.contains(coordinate))
+            .map(|g| g.color)
+    }
+
+    pub fn group(&self, coordinate: &Coordinate) -> Option<&Group> {
+        self.groups
+            .iter()
+            .find(|g| g.coordinates.contains(coordinate))
+    }
+
+    pub fn remove_group(&mut self, group: &Group) {
+        self.groups.retain(|g| g != group);
+    }
+
+    pub fn adjacent_groups(&self, group: &Group) -> Vec<Group> {
+        let adjacent_spaces = self.group_halo(group).into_iter();
+        let mut grps: Vec<Group> = Vec::new();
+
+        adjacent_spaces.for_each(|coord| match self.group(&coord) {
+            None => return,
+            Some(adj) => {
+                if group.color == adj.color {
+                    return;
+                }
+                if grps.iter().any(|g| g.coordinates.contains(&coord)) {
+                    return;
+                }
+                grps.push(adj.clone());
+            }
+        });
+
+        grps
+    }
+
+    pub fn group_halo(&self, group: &Group) -> HashSet<Coordinate> {
+        group
+            .coordinates
+            .iter()
+            .map(|c| self.adjacencies(c))
+            .flatten()
+            .collect::<HashSet<Coordinate>>()
+    }
+
+    pub fn liberties(&self, group: &Group) -> usize {
+        self.group_halo(group)
+            .into_iter()
+            .filter(|c| self.stone(&c) == None)
+            .count()
+    }
+
+    pub fn adjacencies(&self, coordinate: &Coordinate) -> Vec<Coordinate> {
+        let mut v = Vec::new();
+        if coordinate.column > 0 {
+            v.push(Coordinate {
+                column: coordinate.column - 1,
+                row: coordinate.row,
+            });
+        }
+        if coordinate.row > 0 {
+            v.push(Coordinate {
+                column: coordinate.column,
+                row: coordinate.row - 1,
+            });
+        }
+        v.push(Coordinate {
+            column: coordinate.column + 1,
+            row: coordinate.row,
+        });
+        v.push(Coordinate {
+            column: coordinate.column,
+            row: coordinate.row + 1,
+        });
+        v.into_iter().filter(|c| self.within_board(c)).collect()
+    }
+
+    pub fn within_board(&self, coordinate: &Coordinate) -> bool {
+        coordinate.column < self.size.width && coordinate.row < self.size.height
+    }
+}
+
+#[derive(Clone, Debug, PartialEq)]
+pub struct Group {
+    color: Color,
+    coordinates: HashSet<Coordinate>,
+}
+
+impl Group {
+    fn contains(&self, coordinate: &Coordinate) -> bool {
+        self.coordinates.contains(coordinate)
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use super::*;
+
+    /* Two players (Black and White) take turns and Black plays first
+     * Stones are placed on the line intersections and not moved.
+     * A stone with no liberties is removed from the board.
+     * A group of stones of the same color share liberties.
+     * A stone at the edge of the board has only three liberties.
+     * A stone at the corner of the board has only two liberties.
+     * A stone may not be placed in a suicidal position.
+     * A stone placed in a suicidal position is legal if it captures other stones first.
+     */
+
+    fn with_example_board(test: impl FnOnce(Board)) {
+        let board = Board::from_coordinates(
+            vec![
+                (Coordinate { column: 3, row: 3 }, Color::White),
+                (Coordinate { column: 3, row: 4 }, Color::White),
+                /* */
+                (Coordinate { column: 8, row: 3 }, Color::Black),
+                (Coordinate { column: 9, row: 3 }, Color::Black),
+                (Coordinate { column: 9, row: 4 }, Color::Black),
+                /* */
+                (Coordinate { column: 15, row: 3 }, Color::White),
+                (Coordinate { column: 15, row: 4 }, Color::White),
+                (Coordinate { column: 15, row: 5 }, Color::White),
+                (Coordinate { column: 14, row: 4 }, Color::White),
+                /* */
+                (Coordinate { column: 3, row: 8 }, Color::White),
+                (Coordinate { column: 3, row: 9 }, Color::White),
+                (Coordinate { column: 4, row: 9 }, Color::White),
+                (Coordinate { column: 3, row: 10 }, Color::Black),
+                /* */
+                (Coordinate { column: 0, row: 0 }, Color::White),
+                (Coordinate { column: 1, row: 0 }, Color::White),
+                (Coordinate { column: 0, row: 1 }, Color::White),
+                /* */
+                (Coordinate { column: 9, row: 9 }, Color::White),
+                (Coordinate { column: 8, row: 9 }, Color::Black),
+                (Coordinate { column: 9, row: 8 }, Color::Black),
+                (Coordinate { column: 9, row: 10 }, Color::Black),
+                /* */
+                (Coordinate { column: 0, row: 17 }, Color::White),
+                (Coordinate { column: 1, row: 18 }, Color::White),
+                (Coordinate { column: 0, row: 18 }, Color::White),
+                (Coordinate { column: 0, row: 16 }, Color::Black),
+                (Coordinate { column: 1, row: 17 }, Color::Black),
+                /* */
+                (Coordinate { column: 4, row: 17 }, Color::Black),
+                (Coordinate { column: 5, row: 17 }, Color::Black),
+                (Coordinate { column: 6, row: 17 }, Color::Black),
+                (Coordinate { column: 4, row: 18 }, Color::Black),
+                (Coordinate { column: 6, row: 18 }, Color::Black),
+                (Coordinate { column: 3, row: 17 }, Color::White),
+                (Coordinate { column: 3, row: 18 }, Color::White),
+                (Coordinate { column: 4, row: 16 }, Color::White),
+                (Coordinate { column: 5, row: 16 }, Color::White),
+                (Coordinate { column: 6, row: 16 }, Color::White),
+                (Coordinate { column: 7, row: 17 }, Color::White),
+                (Coordinate { column: 7, row: 18 }, Color::White),
+                /* */
+                (Coordinate { column: 17, row: 0 }, Color::White),
+                (Coordinate { column: 17, row: 1 }, Color::White),
+                (Coordinate { column: 18, row: 1 }, Color::White),
+            ]
+            .into_iter(),
+        )
+        .unwrap();
+        test(board);
+    }
+
+    #[test]
+    fn it_gets_adjacencies_for_coordinate() {
+        let board = Board::new();
+        for column in 0..19 {
+            for row in 0..19 {
+                for coordinate in board.adjacencies(&Coordinate { column, row }) {
+                    assert!(
+                        board.within_board(&coordinate),
+                        "{} {}: {:?}",
+                        column,
+                        row,
+                        coordinate
+                    );
+                }
+            }
+        }
+    }
+
+    #[test]
+    fn it_counts_individual_liberties() {
+        let board = Board::from_coordinates(
+            vec![
+                (Coordinate { column: 3, row: 3 }, Color::White),
+                (Coordinate { column: 0, row: 3 }, Color::White),
+                (Coordinate { column: 0, row: 0 }, Color::White),
+                (Coordinate { column: 18, row: 9 }, Color::Black),
+                (
+                    Coordinate {
+                        column: 18,
+                        row: 18,
+                    },
+                    Color::Black,
+                ),
+            ]
+            .into_iter(),
+        )
+        .unwrap();
+        assert!(board.group(&Coordinate { column: 18, row: 3 }).is_none());
+        assert_eq!(
+            board
+                .group(&Coordinate { column: 3, row: 3 })
+                .map(|g| board.liberties(&g)),
+            Some(4)
+        );
+        assert_eq!(
+            board
+                .group(&Coordinate { column: 0, row: 3 })
+                .map(|g| board.liberties(&g)),
+            Some(3)
+        );
+        assert_eq!(
+            board
+                .group(&Coordinate { column: 0, row: 0 })
+                .map(|g| board.liberties(&g)),
+            Some(2)
+        );
+        assert_eq!(
+            board
+                .group(&Coordinate { column: 18, row: 9 })
+                .map(|g| board.liberties(&g)),
+            Some(3)
+        );
+        assert_eq!(
+            board
+                .group(&Coordinate {
+                    column: 18,
+                    row: 18
+                })
+                .map(|g| board.liberties(&g)),
+            Some(2)
+        );
+    }
+
+    #[test]
+    fn stones_share_liberties() {
+        with_example_board(|board: Board| {
+            let test_cases = vec![
+                (
+                    board.clone(),
+                    Coordinate { column: 0, row: 0 },
+                    Some(Group {
+                        color: Color::White,
+                        coordinates: vec![
+                            Coordinate { column: 0, row: 0 },
+                            Coordinate { column: 1, row: 0 },
+                            Coordinate { column: 0, row: 1 },
+                        ]
+                        .into_iter()
+                        .collect(),
+                    }),
+                    Some(3),
+                ),
+                (
+                    board.clone(),
+                    Coordinate { column: 1, row: 0 },
+                    Some(Group {
+                        color: Color::White,
+                        coordinates: vec![
+                            Coordinate { column: 0, row: 0 },
+                            Coordinate { column: 1, row: 0 },
+                            Coordinate { column: 0, row: 1 },
+                        ]
+                        .into_iter()
+                        .collect(),
+                    }),
+                    Some(3),
+                ),
+                (
+                    board.clone(),
+                    Coordinate { column: 9, row: 9 },
+                    Some(Group {
+                        color: Color::White,
+                        coordinates: vec![Coordinate { column: 9, row: 9 }].into_iter().collect(),
+                    }),
+                    Some(1),
+                ),
+                (
+                    board.clone(),
+                    Coordinate { column: 3, row: 4 },
+                    Some(Group {
+                        color: Color::White,
+                        coordinates: vec![
+                            Coordinate { column: 3, row: 3 },
+                            Coordinate { column: 3, row: 4 },
+                        ]
+                        .into_iter()
+                        .collect(),
+                    }),
+                    Some(6),
+                ),
+                (
+                    board.clone(),
+                    Coordinate { column: 9, row: 3 },
+                    Some(Group {
+                        color: Color::Black,
+                        coordinates: vec![
+                            Coordinate { column: 8, row: 3 },
+                            Coordinate { column: 9, row: 3 },
+                            Coordinate { column: 9, row: 4 },
+                        ]
+                        .into_iter()
+                        .collect(),
+                    }),
+                    Some(7),
+                ),
+                (
+                    board.clone(),
+                    Coordinate { column: 15, row: 4 },
+                    Some(Group {
+                        color: Color::White,
+                        coordinates: vec![
+                            Coordinate { column: 15, row: 3 },
+                            Coordinate { column: 15, row: 4 },
+                            Coordinate { column: 15, row: 5 },
+                            Coordinate { column: 14, row: 4 },
+                        ]
+                        .into_iter()
+                        .collect(),
+                    }),
+                    Some(8),
+                ),
+                (
+                    board.clone(),
+                    Coordinate { column: 3, row: 9 },
+                    Some(Group {
+                        color: Color::White,
+                        coordinates: vec![
+                            Coordinate { column: 3, row: 8 },
+                            Coordinate { column: 3, row: 9 },
+                            Coordinate { column: 4, row: 9 },
+                        ]
+                        .into_iter()
+                        .collect(),
+                    }),
+                    Some(6),
+                ),
+                (
+                    board.clone(),
+                    Coordinate { column: 0, row: 18 },
+                    Some(Group {
+                        color: Color::White,
+                        coordinates: vec![
+                            Coordinate { column: 0, row: 17 },
+                            Coordinate { column: 0, row: 18 },
+                            Coordinate { column: 1, row: 18 },
+                        ]
+                        .into_iter()
+                        .collect(),
+                    }),
+                    Some(1),
+                ),
+                (
+                    board.clone(),
+                    Coordinate { column: 0, row: 17 },
+                    Some(Group {
+                        color: Color::White,
+                        coordinates: vec![
+                            Coordinate { column: 0, row: 17 },
+                            Coordinate { column: 0, row: 18 },
+                            Coordinate { column: 1, row: 18 },
+                        ]
+                        .into_iter()
+                        .collect(),
+                    }),
+                    Some(1),
+                ),
+            ];
+
+            println!("{}", board);
+            for (board, coordinate, group, liberties) in test_cases {
+                assert_eq!(board.group(&coordinate), group.as_ref());
+                assert_eq!(
+                    board.group(&coordinate).map(|g| board.liberties(&g)),
+                    liberties,
+                    "{:?}",
+                    coordinate
+                );
+            }
+        });
+    }
+
+    #[test]
+    fn it_finds_adjacent_groups() {
+        with_example_board(|board| {
+            let group = board
+                .group(&Coordinate { column: 0, row: 0 })
+                .cloned()
+                .unwrap();
+            assert_eq!(board.adjacent_groups(&group), Vec::new());
+
+            let group = board
+                .group(&Coordinate { column: 3, row: 10 })
+                .cloned()
+                .unwrap();
+            assert_eq!(board.adjacent_groups(&group).len(), 1);
+        });
+    }
+
+    #[test]
+    fn surrounding_a_group_removes_it() {
+        with_example_board(|board| {
+            let board = board
+                .place_stone(Coordinate { column: 10, row: 9 }, Color::Black)
+                .unwrap();
+            assert!(board.stone(&Coordinate { column: 9, row: 9 }).is_none());
+
+            let board = board
+                .place_stone(Coordinate { column: 2, row: 18 }, Color::Black)
+                .unwrap();
+            assert!(board.stone(&Coordinate { column: 0, row: 18 }).is_none());
+            assert!(board.stone(&Coordinate { column: 1, row: 18 }).is_none());
+            assert!(board.stone(&Coordinate { column: 0, row: 17 }).is_none());
+            assert!(board.group(&Coordinate { column: 0, row: 18 }).is_none());
+
+            let board = board
+                .place_stone(Coordinate { column: 5, row: 18 }, Color::White)
+                .unwrap();
+            assert!(board.stone(&Coordinate { column: 4, row: 17 }).is_none());
+            assert!(board.stone(&Coordinate { column: 5, row: 17 }).is_none());
+            assert!(board.stone(&Coordinate { column: 6, row: 17 }).is_none());
+            assert!(board.stone(&Coordinate { column: 4, row: 18 }).is_none());
+            assert!(board.stone(&Coordinate { column: 6, row: 18 }).is_none());
+        });
+    }
+
+    #[test]
+    fn self_capture_is_forbidden() {
+        with_example_board(|board| {
+            {
+                let board = board.clone();
+                let res = board.place_stone(Coordinate { column: 18, row: 0 }, Color::Black);
+                assert_eq!(res, Err(BoardError::SelfCapture));
+            }
+
+            {
+                let board = board.clone();
+                let res = board.place_stone(Coordinate { column: 5, row: 18 }, Color::Black);
+                assert_eq!(res, Err(BoardError::SelfCapture));
+            }
+        });
+    }
+
+    #[test]
+    fn validate_group_comparisons() {
+        {
+            let b1 = Board::from_coordinates(
+                vec![(Coordinate { column: 7, row: 9 }, Color::White)].into_iter(),
+            )
+            .unwrap();
+            let b2 = Board::from_coordinates(
+                vec![(Coordinate { column: 7, row: 9 }, Color::White)].into_iter(),
+            )
+            .unwrap();
+
+            assert_eq!(b1, b2);
+        }
+
+        {
+            let b1 = Board::from_coordinates(
+                vec![
+                    (Coordinate { column: 7, row: 9 }, Color::White),
+                    (Coordinate { column: 8, row: 10 }, Color::White),
+                ]
+                .into_iter(),
+            )
+            .unwrap();
+            let b2 = Board::from_coordinates(
+                vec![
+                    (Coordinate { column: 8, row: 10 }, Color::White),
+                    (Coordinate { column: 7, row: 9 }, Color::White),
+                ]
+                .into_iter(),
+            )
+            .unwrap();
+
+            assert_eq!(b1, b2);
+        }
+    }
+
+    #[test]
+    fn two_boards_can_be_compared() {
+        let board = Board::from_coordinates(
+            vec![
+                (Coordinate { column: 7, row: 9 }, Color::White),
+                (Coordinate { column: 8, row: 8 }, Color::White),
+                (Coordinate { column: 8, row: 10 }, Color::White),
+                (Coordinate { column: 9, row: 9 }, Color::White),
+                (Coordinate { column: 10, row: 9 }, Color::Black),
+                (Coordinate { column: 9, row: 8 }, Color::Black),
+                (Coordinate { column: 9, row: 10 }, Color::Black),
+            ]
+            .into_iter(),
+        )
+        .unwrap();
+
+        let b1 = board
+            .clone()
+            .place_stone(Coordinate { column: 8, row: 9 }, Color::Black)
+            .unwrap();
+
+        let b2 = b1
+            .clone()
+            .place_stone(Coordinate { column: 9, row: 9 }, Color::White)
+            .unwrap();
+
+        assert_eq!(board, b2);
+    }
+}
diff --git a/kifu/kifu-core/src/lib.rs b/kifu/kifu-core/src/lib.rs
index 9c22bc8..8b598bb 100644
--- a/kifu/kifu-core/src/lib.rs
+++ b/kifu/kifu-core/src/lib.rs
@@ -2,5 +2,8 @@ mod api;
 pub use api::{CoreApp, Request, Response};
 
 mod types;
-pub use types::{Color, Size};
+pub use types::{BoardError, Color, Size};
 pub mod ui;
+
+mod board;
+pub use board::*;
diff --git a/kifu/kifu-core/src/types.rs b/kifu/kifu-core/src/types.rs
index e9c9f32..912632f 100644
--- a/kifu/kifu-core/src/types.rs
+++ b/kifu/kifu-core/src/types.rs
@@ -1,16 +1,30 @@
-use crate::api::PlayStoneRequest;
+use crate::{
+    api::PlayStoneRequest,
+    board::{Board, Coordinate},
+};
 use std::time::Duration;
+use thiserror::Error;
 
-#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+#[derive(Debug, PartialEq, Error)]
+pub enum BoardError {
+    #[error("Position is invalid")]
+    InvalidPosition,
+    #[error("Self-capture is forbidden")]
+    SelfCapture,
+    #[error("Ko")]
+    Ko,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Hash, Eq)]
 pub enum Color {
     Black,
     White,
 }
 
-#[derive(Clone, Copy, Debug)]
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub struct Size {
-    pub width: u8,
-    pub height: u8,
+    pub width: usize,
+    pub height: usize,
 }
 
 impl Default for Size {
@@ -36,7 +50,10 @@ impl AppState {
     pub fn place_stone(&mut self, req: PlayStoneRequest) {
         match self.game {
             Some(ref mut game) => {
-                game.place_stone(req.column, req.row);
+                game.place_stone(Coordinate {
+                    column: req.column,
+                    row: req.row,
+                });
             }
             None => {}
         }
@@ -74,6 +91,8 @@ pub struct Player {
 
 pub struct GameState {
     pub board: Board,
+    pub past_positions: Vec<Board>,
+
     pub conversation: Vec<String>,
     pub current_player: Color,
 
@@ -88,6 +107,7 @@ impl GameState {
     fn new() -> GameState {
         GameState {
             board: Board::new(),
+            past_positions: vec![],
             conversation: vec![],
             current_player: Color::Black,
             white_player: Player {
@@ -103,46 +123,78 @@ impl GameState {
         }
     }
 
-    fn place_stone(&mut self, column: u8, row: u8) {
-        self.board.place_stone(column, row, self.current_player);
+    fn place_stone(&mut self, coordinate: Coordinate) -> Result<(), BoardError> {
+        let board = self.board.clone();
+        let new_board = board.place_stone(coordinate, self.current_player)?;
+
+        if self.past_positions.contains(&new_board) {
+            return Err(BoardError::Ko);
+        }
+
+        self.past_positions.push(self.board.clone());
+        self.board = new_board;
         match self.current_player {
             Color::White => self.current_player = Color::Black,
             Color::Black => self.current_player = Color::White,
-        }
+        };
+        Ok(())
     }
 }
 
-pub struct Board {
-    pub size: Size,
-    pub spaces: Vec<Option<Color>>,
-}
+#[cfg(test)]
+mod test {
+    use super::*;
 
-impl Board {
-    fn new() -> Self {
-        let mut spaces = Vec::new();
-        for _ in 0..19 * 19 {
-            spaces.push(None);
-        }
-        Self {
-            size: Size {
-                width: 19,
-                height: 19,
-            },
-            spaces,
-        }
+    #[test]
+    fn current_player_changes_after_move() {
+        let mut state = GameState::new();
+        assert_eq!(state.current_player, Color::Black);
+        state.place_stone(Coordinate { column: 9, row: 9 }).unwrap();
+        assert_eq!(state.current_player, Color::White);
     }
 
-    pub fn place_stone(&mut self, column: u8, row: u8, stone: Color) {
-        let addr = self.addr(column, row);
-        self.spaces[addr] = Some(stone);
+    #[test]
+    fn current_player_remains_the_same_after_self_capture() {
+        let mut state = GameState::new();
+        state.board = Board::from_coordinates(
+            vec![
+                (Coordinate { column: 17, row: 0 }, Color::White),
+                (Coordinate { column: 17, row: 1 }, Color::White),
+                (Coordinate { column: 18, row: 1 }, Color::White),
+            ]
+            .into_iter(),
+        )
+        .unwrap();
+        state.current_player = Color::Black;
+
+        assert_eq!(
+            state.place_stone(Coordinate { column: 18, row: 0 }),
+            Err(BoardError::SelfCapture)
+        );
+        assert_eq!(state.current_player, Color::Black);
     }
 
-    pub fn stone(&self, column: u8, row: u8) -> Option<Color> {
-        let addr = self.addr(column, row);
-        self.spaces[addr]
-    }
+    #[test]
+    fn ko_rules_are_enforced() {
+        let mut state = GameState::new();
+        state.board = Board::from_coordinates(
+            vec![
+                (Coordinate { column: 7, row: 9 }, Color::White),
+                (Coordinate { column: 8, row: 8 }, Color::White),
+                (Coordinate { column: 8, row: 10 }, Color::White),
+                (Coordinate { column: 9, row: 9 }, Color::White),
+                (Coordinate { column: 10, row: 9 }, Color::Black),
+                (Coordinate { column: 9, row: 8 }, Color::Black),
+                (Coordinate { column: 9, row: 10 }, Color::Black),
+            ]
+            .into_iter(),
+        )
+        .unwrap();
 
-    fn addr(&self, column: u8, row: u8) -> usize {
-        ((row as usize) * (self.size.width as usize) + (column as usize)) as usize
+        state.place_stone(Coordinate { column: 8, row: 9 }).unwrap();
+        assert_eq!(
+            state.place_stone(Coordinate { column: 9, row: 9 }),
+            Err(BoardError::Ko)
+        );
     }
 }
diff --git a/kifu/kifu-core/src/ui/playing_field.rs b/kifu/kifu-core/src/ui/playing_field.rs
index cdc9c12..abd5456 100644
--- a/kifu/kifu-core/src/ui/playing_field.rs
+++ b/kifu/kifu-core/src/ui/playing_field.rs
@@ -1,5 +1,7 @@
-use crate::types::Color;
-use crate::{types::GameState, ui::types};
+use crate::{
+    types::{Color, GameState},
+    ui::types,
+};
 
 #[derive(Clone, Debug)]
 pub struct PlayingFieldView {
diff --git a/kifu/kifu-core/src/ui/types.rs b/kifu/kifu-core/src/ui/types.rs
index 39e2904..027f8db 100644
--- a/kifu/kifu-core/src/ui/types.rs
+++ b/kifu/kifu-core/src/ui/types.rs
@@ -1,7 +1,7 @@
 use crate::types::{Color, Size};
 use crate::{
     api::{PlayStoneRequest, Request},
-    types::Board,
+    Board, Coordinate,
 };
 
 #[derive(Clone, Copy, Debug, PartialEq)]
@@ -14,12 +14,14 @@ pub struct Jitter {
 pub struct StoneElement {
     pub color: Color,
     pub jitter: Jitter,
+    pub liberties: Option<u8>,
 }
 
 impl StoneElement {
-    pub fn new(color: Color) -> Self {
+    pub fn new(color: Color, liberties: Option<u8>) -> Self {
         Self {
             color,
+            liberties,
             jitter: Jitter { x: 0, y: 0 },
         }
     }
@@ -71,9 +73,10 @@ impl From<&Board> for BoardElement {
         let spaces: Vec<IntersectionElement> = (0..board.size.height)
             .map(|row| {
                 (0..board.size.width)
-                    .map(|column| match board.stone(column, row) {
+                    .map(|column| match board.stone(&Coordinate { column, row }) {
                         Some(color) => IntersectionElement::Filled(StoneElement {
                             jitter: Jitter { x: 0, y: 0 },
+                            liberties: None,
                             color,
                         }),
                         None => IntersectionElement::Empty(Request::PlayStoneRequest(
diff --git a/kifu/kifu-gtk/Cargo.lock b/kifu/kifu-gtk/Cargo.lock
index 7e5c456..c7faf28 100644
--- a/kifu/kifu-gtk/Cargo.lock
+++ b/kifu/kifu-gtk/Cargo.lock
@@ -488,6 +488,15 @@ dependencies = [
  "system-deps",
 ]
 
+[[package]]
+name = "grid"
+version = "0.9.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0634107a3a005070dd73e27e74ecb691a94e9e5ba7829f434db7fbf73a6b5c47"
+dependencies = [
+ "no-std-compat",
+]
+
 [[package]]
 name = "gsk4"
 version = "0.6.3"
@@ -657,6 +666,8 @@ dependencies = [
 name = "kifu-core"
 version = "0.1.0"
 dependencies = [
+ "grid",
+ "thiserror",
  "tokio",
 ]
 
@@ -751,6 +762,12 @@ dependencies = [
  "getrandom",
 ]
 
+[[package]]
+name = "no-std-compat"
+version = "0.4.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b93853da6d84c2e3c7d730d6473e8817692dd89be387eb01b94d7f108ecb5b8c"
+
 [[package]]
 name = "num-integer"
 version = "0.1.45"
diff --git a/kifu/kifu-gtk/Makefile b/kifu/kifu-gtk/Makefile
new file mode 100644
index 0000000..2297add
--- /dev/null
+++ b/kifu/kifu-gtk/Makefile
@@ -0,0 +1,5 @@
+release:
+	cargo build --release
+
+dev:
+	cargo watch -x 'run --bin kifu-gtk'
diff --git a/kifu/kifu-gtk/src/ui/board.rs b/kifu/kifu-gtk/src/ui/board.rs
index 7b442f5..2940066 100644
--- a/kifu/kifu-gtk/src/ui/board.rs
+++ b/kifu/kifu-gtk/src/ui/board.rs
@@ -141,7 +141,7 @@ impl ObjectImpl for BoardPrivate {
                     (0..19).for_each(|row| {
                         match board.stone(row, col) {
                             IntersectionElement::Filled(stone) => {
-                                pen.stone(&context, row, col, stone.color);
+                                pen.stone(&context, row, col, stone.color, stone.liberties);
                             }
                             _ => {}
                         };
@@ -270,12 +270,27 @@ impl Pen {
         let _ = context.fill();
     }
 
-    fn stone(&self, context: &cairo::Context, row: u8, col: u8, color: Color) {
+    fn stone(
+        &self,
+        context: &cairo::Context,
+        row: u8,
+        col: u8,
+        color: Color,
+        liberties: Option<u8>,
+    ) {
         match color {
             Color::White => context.set_source_rgb(0.9, 0.9, 0.9),
             Color::Black => context.set_source_rgb(0.0, 0.0, 0.0),
         };
         self.draw_stone(context, row, col);
+
+        if let Some(liberties) = liberties {
+            let stone_location = self.stone_location(row, col);
+            context.set_source_rgb(1., 0., 1.);
+            context.set_font_size(32.);
+            context.move_to(stone_location.0 - 10., stone_location.1 + 10.);
+            let _ = context.show_text(&format!("{}", liberties));
+        }
     }
 
     fn ghost_stone(&self, context: &cairo::Context, row: u8, col: u8, color: Color) {
@@ -288,13 +303,15 @@ impl Pen {
 
     fn draw_stone(&self, context: &cairo::Context, row: u8, col: u8) {
         let radius = self.hspace_between / 2. - 2.;
-        context.arc(
-            self.x_offset + (col as f64) * self.hspace_between,
-            self.y_offset + (row as f64) * self.vspace_between,
-            radius,
-            0.0,
-            2.0 * std::f64::consts::PI,
-        );
+        let (x_loc, y_loc) = self.stone_location(row, col);
+        context.arc(x_loc, y_loc, radius, 0.0, 2.0 * std::f64::consts::PI);
         let _ = context.fill();
     }
+
+    fn stone_location(&self, row: u8, col: u8) -> (f64, f64) {
+        (
+            self.x_offset + (col as f64) * self.hspace_between,
+            self.y_offset + (row as f64) * self.vspace_between,
+        )
+    }
 }