diff --git a/pixelgl/attr.go b/pixelgl/attr.go
index 16128bc..4f649df 100644
--- a/pixelgl/attr.go
+++ b/pixelgl/attr.go
@@ -20,6 +20,8 @@ const (
 	Transform
 	// Camera is a camera view matrix
 	Camera
+	// NumStandardAttrPurposes is the number of standard attribute purposes
+	NumStandardAttrPurposes
 )
 
 // AttrType represents the type of an OpenGL attribute.