diff --git a/spatial/r2/vector.go b/spatial/r2/vector.go
index b7b9cd6d..3419ed55 100644
--- a/spatial/r2/vector.go
+++ b/spatial/r2/vector.go
@@ -30,6 +30,11 @@ func (p Vec) Scale(f float64) Vec {
 	return p
 }
 
+// Dot returns the dot product p·q.
+func (p Vec) Dot(q Vec) float64 {
+	return p.X*q.X + p.Y*q.Y
+}
+
 // Box is a 2D bounding box.
 type Box struct {
 	Min, Max Vec
diff --git a/spatial/r2/vector_test.go b/spatial/r2/vector_test.go
index 09081218..3ccc2c7e 100644
--- a/spatial/r2/vector_test.go
+++ b/spatial/r2/vector_test.go
@@ -77,3 +77,38 @@ func TestScale(t *testing.T) {
 		})
 	}
 }
+
+func TestDot(t *testing.T) {
+	for _, test := range []struct {
+		u, v Vec
+		want float64
+	}{
+		{Vec{1, 2}, Vec{1, 2}, 5},
+		{Vec{1, 0}, Vec{1, 0}, 1},
+		{Vec{1, 0}, Vec{0, 1}, 0},
+		{Vec{1, 0}, Vec{0, 1}, 0},
+		{Vec{1, 1}, Vec{-1, -1}, -2},
+		{Vec{1, 2}, Vec{-0.3, 0.4}, 0.5},
+	} {
+		t.Run("", func(t *testing.T) {
+			{
+				got := test.u.Dot(test.v)
+				if got != test.want {
+					t.Fatalf(
+						"error: %v · %v: got=%v, want=%v",
+						test.u, test.v, got, test.want,
+					)
+				}
+			}
+			{
+				got := test.v.Dot(test.u)
+				if got != test.want {
+					t.Fatalf(
+						"error: %v · %v: got=%v, want=%v",
+						test.v, test.u, got, test.want,
+					)
+				}
+			}
+		})
+	}
+}