What is the difference between tapered and parallel threads? One potential way to distinguish between thread types is to determine whether the thread is tapered or parallel. You can see the different between these two types by looking along the length of the thread. A thread that is tapered will narrow in its diameter across the entire length of the part. Alternatively, a thread that is parallel will remain the exact same diameter across the part’s length. Whether a thread is tapered or parallel can typically be determined by just looking at it, however when the difference is minute you can use a caliper to make measurements of the diameter along different points. Knowing whether the thread you are working with is tapered or parallel will help you to find the perfect fit into a corresponding part.