diff --git a/src/card.rs b/src/card.rs
new file mode 100644
index 0000000..e91f728
--- /dev/null
+++ b/src/card.rs
@@ -0,0 +1,157 @@
+//! TODO: module doc :v
+
+/// Value for the "sub tiles" inside a room tile
+#[derive(Clone, Copy, Debug, Default, PartialEq)]
+enum Cell {
+  #[default]
+  Empty,
+  NW,
+  NE,
+  SE,
+  SW,
+  Filled,
+}
+
+#[derive(Clone, Copy, Debug)]
+enum CutLine {
+  VertLeft,
+  VertRight,
+  HorizUpper,
+  HorizLower,
+}
+
+#[derive(Clone, Copy, Debug)]
+enum FlipDir {
+  Vertical,
+  Horizontal,
+}
+
+#[derive(Clone, Copy, Debug)]
+enum TransposeIndex {
+  First,
+  Second,
+  Third,
+}
+
+#[derive(Clone, Copy, Debug)]
+enum TransposeSelection {
+  Column(TransposeIndex),
+  Row(TransposeIndex),
+}
+
+#[derive(Clone, Copy, Debug)]
+enum RotationDir {
+  Clockwise,
+  CounterClockwise,
+}
+
+/// An invidiual room, or "card" in the player's hand. The room may
+/// *or may not* be valid, yet.
+#[derive(Clone, Copy, Debug, Default, PartialEq)]
+struct Card {
+  cells: [Cell; 9],
+  e: bool,
+  ne: bool,
+  n: bool,
+  nw: bool,
+  w: bool,
+  sw: bool,
+  s: bool,
+  se: bool,
+}
+
+impl Card {
+  /// Produces a new card by stacking another on top of this one.
+  pub fn merge(self, top: Self) -> (Self, Option<Self>) {
+    todo!();
+  }
+
+  /// Cuts this Card on the given line. Returns two cards
+  pub fn cut(&self, line: CutLine) -> (Self, Self) {
+    todo!();
+  }
+
+  pub fn flip(&self, flip: FlipDir) -> (Self) {
+    todo!();
+  }
+
+  pub fn transpose(&self, other: &Self, selection: TransposeSelection) -> (Self, Self) {
+    todo!();
+  }
+
+  pub fn rotate(&self, dir: RotationDir) {
+    todo!();
+  }
+}
+
+#[rustfmt::skip]
+pub const FULL_SQUARE: [Cell; 9] = [
+  Cell::Filled, Cell::Filled, Cell::Filled,
+  Cell::Filled, Cell::Filled, Cell::Filled,
+  Cell::Filled, Cell::Filled, Cell::Filled,
+];
+
+#[rustfmt::skip]
+pub const NW_TRIANGLE: [Cell; 9] = [
+  Cell::Empty, Cell::Empty, Cell::NW,
+  Cell::Empty, Cell::NW, Cell::Filled,
+  Cell::NW, Cell::Filled, Cell::Filled,
+];
+
+#[rustfmt::skip]
+pub const OCTAGON: [Cell; 9] = [
+  Cell::NW, Cell::Filled, Cell::NE,
+  Cell::Filled, Cell::Filled, Cell::Filled,
+  Cell::SW, Cell::Filled, Cell::SE,
+];
+
+#[cfg(test)]
+mod test {
+  use super::*;
+
+  #[test]
+  fn check_merge() {
+    let bottom = Card {
+      cells: NW_TRIANGLE,
+      ..Default::default() // no doors
+    };
+
+    let top = Card {
+      cells: OCTAGON,
+      ..Default::default()
+    };
+
+    // For now, triangular cells from the top-most Card survive the merge.
+    let expected = Card {
+      cells: [
+        Cell::NW, Cell::Filled, Cell::NE,
+        Cell::Filled, Cell::Filled, Cell::Filled,
+        Cell::SW, Cell::Filled, Cell::Filled,
+      ],
+      ..Default::default()
+    };
+
+    // Run the test
+    let (stacked, extra_doors) = bottom.merge(top);
+    assert_eq!(stacked, expected);
+    assert!(extra_doors.is_none());
+  }
+
+  /// Merging two corner cells together results in the upper cell coming through.
+  #[test]
+  fn merge_triangle_and_triangle() {
+    todo!();
+  }
+
+  /// Merging a filled cell with anything should result in [`Cell::Filled`].
+  #[test]
+  fn merge_triangle_and_filled() {
+    todo!();
+  }
+
+  /// Merging a NW and SE cell should result in a single [`Cell::Filled`].
+  #[test]
+  fn merge_opposite_triangles() {
+    todo!();
+  }
+}
diff --git a/src/main.rs b/src/main.rs
index 826132f..c3ac2fb 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,8 @@
 use bevy::{prelude::*, window::WindowResolution};
 use bevy_inspector_egui::{bevy_egui::EguiPlugin, quick::WorldInspectorPlugin};
 
+mod card;
+
 fn main() {
   App::new()
     .add_plugins(DefaultPlugins.set(WindowPlugin {