Generally, screws have single threads and unless it is designated otherwise, it is fair to assume that a thread will be single. This means that the thread is formed from one continuous helix. The lead of a thread is the distance moved by a mating nut in one complete revolution of the nut. In a single thread the lead is equal to the pitch.
When a two start thread is manufactured, there are two continuous helices and to accommodate the grooves, the lead is twice the thread pitch. Multiple threads are used where a quick axial movement is required with a minimum number of turns.
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