What is meant by Thread Gauge

http://en.wikipedia.org/wiki/Thread_pitch_gauge

Thread Gauges are small steel plates usualy fastened together that show the profile of different threads on bolts and nuts. they are generaly used to identify what type of thead there is on component. You offer gauge and thead up together and if their profiles match read the side of gauge and it should show thread type, UNC, UNF, Whitworth, angle ect