kujutletava silindri läbimõõt, mille pind lõikab keerme profiili võrdsel kaugusel keerme harjast ja soonest ühepoolse baassammu ulatuses, diameter of an imaginary cylinder, which intersects the surface of thread profile in such a manner as to make the width of the thread groove equal to one-half of the basic (nominal) pitch