I created these with Fusion out of the need for a stacked knob set on my Bass. There is a 2.5mm hole on each one to act as a guide for inserts. They are for 6mm smooth shaft potentiometers. The upper knob has a dimple for a posistion indicator. You can fill it with pain or leave it as is. Hope this helps someone . I have some other basic knobs I hope to upload in the future.