This is my first original design. I tried other ratchet holders and alot of them the fit wasn't right. I work in a shop with other techs and this design was the most universal. All three have holes in the back for magnets 5mm deep x 12.8. I got the magnets at home depot in a pack of 40 12.7mmx4.75mm. ½ holder holds ratchets up to 20.72mm, 3/8 17.03, ¼ 13.17