on Screw Threads
The basic profile is the theoretical
profile of the thread. An essential principle is that the
actual profiles of both the nut and bolt threads must never
cross or transgress the theoretical profile. So bolt threads
will always be equal to, or smaller than, the dimensions of
the basic profile. Nut threads will always be equal to, or
greater than, the basic profile. To ensure this in practice,
tolerances and allowances are applied to the basic profile.
This is the topic of the next page.