diff --git a/colormatrix.go b/colormatrix.go
index 554dad97e..a7b8e406a 100644
--- a/colormatrix.go
+++ b/colormatrix.go
@@ -28,6 +28,7 @@ const ColorMatrixDim = 5
 // Before applying a matrix, a color is un-multiplied, and after applying the matrix,
 // the color is multiplied again.
 type ColorMatrix struct {
+	// es represents elements.
 	es [ColorMatrixDim - 1][ColorMatrixDim]float64
 }
 
diff --git a/colormatrix_test.go b/colormatrix_test.go
index cba67ce57..d3677e361 100644
--- a/colormatrix_test.go
+++ b/colormatrix_test.go
@@ -20,8 +20,8 @@ import (
 )
 
 func TestColorIdentity(t *testing.T) {
-	ebiten := ColorMatrixI()
-	got := ebiten.IsIdentity()
+	m := ColorMatrixI()
+	got := m.IsIdentity()
 	want := true
 	if want != got {
 		t.Errorf("matrix.IsIdentity() = %t, want %t", got, want)
diff --git a/geometrymatrix.go b/geometrymatrix.go
index f478860f6..b9ece873d 100644
--- a/geometrymatrix.go
+++ b/geometrymatrix.go
@@ -23,6 +23,7 @@ const GeometryMatrixDim = 3
 
 // A GeometryMatrix represents a matrix to transform geometry when rendering an image.
 type GeometryMatrix struct {
+	// es represents elements.
 	es [GeometryMatrixDim - 1][GeometryMatrixDim]float64
 }
 
diff --git a/geometrymatrix_test.go b/geometrymatrix_test.go
index 9251430ff..7b4325da3 100644
--- a/geometrymatrix_test.go
+++ b/geometrymatrix_test.go
@@ -20,8 +20,8 @@ import (
 )
 
 func TestGeometryIdentity(t *testing.T) {
-	ebiten := GeometryMatrixI()
-	got := ebiten.IsIdentity()
+	m := GeometryMatrixI()
+	got := m.IsIdentity()
 	want := true
 	if want != got {
 		t.Errorf("matrix.IsIdentity() = %t, want %t", got, want)