These different sized hand-operated taps and dies are tools used to create screw threads. They illustrate the two types of tool to create either external or internal threads. For the internal threading of a hole of a specific diameter the tap is used, whilst an external thread in a rod is made with the appropriate die for the rod diameter. Taps and dies vary according to the size and pitch of the screw threads required. The three basic types of taps differ on the number of threads which are tapered.