Metric threads are given as pitch in millimetres.
D.P. threads – are you really sure you want to know 
'D.P. threads' are those which engage with a gear wheel of the same D.P. or 'diametric pitch'. D.P. is defined as the 'number of teeth per inch of pitch circle diameter'.
So, for example, a 20 DP gear of any diameter has 20 teeth for every inch of its pitch circle diameter (a circle passing roughly half way through each tooth face used as the base for all gear calculations and is the size that the gear would be if all teh gears in a train were replaced by plain wheels, just touching.
A 40-tooth 20DP gear will have 40 teeth, for example.
How do we make a screw thread to engage with such a gear? (assuming the gear has helical teeth to allow it to engage in line with the screw!) A 20 tooth gear has a 1 inch PCD and therefore a circumference of PI. The pitch of the teeth will therefore be pi/20.
So that's your answer DP threads have a pitch of pi/D.P. and allow threads to be made that will engage with gears. Not a great help as most gears have straight cut teeth and need to engage the screw at its helix angle, and this requires a shorter pitch, which reduces the helix angle requiring a… so it's quite an involve calculation as anyone who reads George Thomas' dividing device for the Myford bull wheel and J.A. Radford's revision of the same will appreciate.
I bet you're really sorry you asked now…
Neil
<and I am praying I got that right because if not the facial egg will be flying thick and fast…>
Edited By Neil Wyatt on 14/07/2014 21:29:51