Although inexpensive, this simple device is a gem for any installer. Just slip the twist drill bit into one of the molded guide-sleeves, and press the sleeve snugly against the surface. The sleeve guarantees that the bit is entering absolutely vertically (what a good thing this is when working over your head). Plus all the drilling debris is captured inside the sleeve. No dust on you, on the floor, on adjacent surfaces – anywhere. Twist drills from 5/32 to 15/32" and from 4 to 12mm will fit one of its 1" deep guide-sleeves. Huge value.