Quad Driver bit holders snap into a quick-release shaft mounted in your drill or driver. The Countersink Set includes a Quad Driver shaft and four bit holders with adjustable drill bits, countersinks and counter-bores for #6, 8, 10 and 12 screws, and magnetic #2 phillips insert bits (which can be replaced with any other hex-shank insert bits) in the opposite end. Four spare drill bits are also provided. A hex wrench for bit adjustment is cleverly housed in the end of the main shaft. The Hardware Installer's Set contains a Quad Driver shaft and three bit holders with self-centering adjustable Vix-type bits on one end, magnetic phillips bits on the other. Drills are sized for #6, 8 and 10 screws.