In the photo above you can see 5 different threads. What makes them different? Below i will explain the many different parts of a thread and how to measure them.
Major Diameter - This is the biggest diameter of the thread. For example a 1/2" - 13 UNC should measure 1/2" on the outside diameter.
The number of threads per inch or in metric the distance between threads. For the thread 3/8" 16 UNC
the 16 number is how many threads per inch. More information click here.
Left or Right hand-
When a nut in put on a thread which way do you have to turn it to loosen?
Most threads are right hand , but some specialty threads are left hand. In applications such as shafts that rotate clock wise a left hand thread will be used so that the nut will not loosen. In the above photo the thread on the far right is left hand. You can tell because the thread slopes down to the right. More information click here.
Is the shape of the thread. Most common threads are 60 degrees V. There are many different forms of threads that all have different applications. Shapes that can be used include Square , Round , 29 degree and tapered. More information click here.
Lead- This is similar to pitch and can be confused between the two. Lead is the distance a nut will travel for one complete turn. Multi start threads will have more lead than a single start thread.