Hopefully you have a much better idea about what a tap and die set is and how it works. Combined, the set can be used to thread or re-thread both male and female portions. In some situations you may only need to use one tool or the other, which is often the case when rethreading. If you are creating a completely new set of threads, then choose a TPI and stick with it. If you are rethreading a single piece, such as bolt, make sure to use the gauge that is included with your tap and die set so that you will know the correct TPI to create.