On Apr 5, 19:54, Merle K. Peirce wrote: In a sense, they don't. They're not normally used for fasteners, which is what the others are for (arguably with the exception of BSP) :-) Yes, they're usually fairly coarse, and used for things like leadscrews on machine tools and vise jaws. For leadscrews, they're usually made to some specific pitch that equates to some nice decimal number of turns per inch, or millimetres per turn. They're also unusual in having large flats on both the crests and troughs (most other threads are rounded on one or the other); in fact they're almost square -- the thread depth is 0.5 x the pitch, and the walls slope at only 14.5 degrees, so the included angle is 29 degree, much less than most threads which are typically either 55 or 60 degrees. And the clearance between screw and nut is very small, typically 0.001". -- Pete Peter Turnbull Network Manager University of York