A thread parallel to the central axis is defined as the line connecting the crests of adjacent threads. 

This type of thread is called a parallel thread, such as a metric thread (M) or a unified thread (U). 

 

A thread inclined to the central axis is called a tapered thread. 

The external thread diameter gradually decreases as it is screwed in, and the minor diameter of the internal thread also decreases. 

Examples include R, Rc, and PT threads. 

 

In addition, there are two types of pipe threads used to connect pipes: parallel threads and tapered threads.