diff --git a/src/card.rs b/src/card.rs
index 1340851..531206a 100644
--- a/src/card.rs
+++ b/src/card.rs
@@ -191,21 +191,61 @@ mod test {
     assert!(extra_doors.is_none());
   }
 
-  /// Merging two corner cells together results in the upper cell coming through.
+  /// Merging two triangular [`Cell`]s should prefer the top-most one, *unless*
+  /// they are opposites. See test [`merge_opposite_triangles()`].
   #[test]
   fn merge_triangle_and_triangle() {
-    todo!();
+    let mut top = Card::default();
+    top.cells[0] = Cell::NW;
+
+    let mut bottom = Card::default();
+    bottom.cells[0] = Cell::NE;
+
+    let expected = top.clone();
+
+    let (stacked, doors) = bottom.merge(top);
+    assert_eq!(stacked, expected);
+    assert!(doors.is_none());
   }
 
   /// Merging a filled cell with anything should result in [`Cell::Filled`].
   #[test]
   fn merge_triangle_and_filled() {
-    todo!();
+    let filled = Card {
+      cells: FULL_SQUARE,
+      ..Default::default()
+    };
+    let triangle = Card {
+      cells: NW_TRIANGLE,
+      ..Default::default()
+    };
+
+    let expected = filled.clone();
+
+    // Check the merge in both directions.
+    let result_fill_over_tri = triangle.merge(filled);
+    let result_tri_over_fill = filled.merge(triangle);
+
+    assert_eq!(expected, result_fill_over_tri.0);
+    assert_eq!(expected, result_tri_over_fill.0);
+    assert!(result_fill_over_tri.1.is_none());
+    assert!(result_tri_over_fill.1.is_none());
   }
 
   /// Merging a NW and SE cell should result in a single [`Cell::Filled`].
   #[test]
   fn merge_opposite_triangles() {
-    todo!();
+    let mut top = Card::default();
+    top.cells[0] = Cell::NW;
+
+    let mut bottom = Card::default();
+    bottom.cells[0] = Cell::SE;
+
+    let mut expected = Card::default();
+    expected.cells[0] = Cell::Filled;
+
+    let (stacked, doors) = bottom.merge(top);
+    assert_eq!(stacked, expected);
+    assert!(doors.is_none());
   }
 }