Gauge for determining drill bit size needed or for determining the size of hole needed for screws, bolts, machine screws etc. Used as a quick crutch instead of getting out the "mic". Or used for sending along with the little lady (with the appropriate opening marked) when she is send to the big box for a hand full of bolts. Of course a real little lady will ask "fine or coarse thread, stainless, brass, etc.?" And if you have been good, probably come back with a six-pack and a can of Skoal too.