machine threads

[J.Harris]

This is mostly correct. In regards to the metric standard the second number doesn't refer to threads per millimeter but rather millimeters per thread. For example an M3-.5 thread has a finer thread than an M8-1.25 . With the M3-.5 a full thread takes up .5 millimeters of length therefore the pitch is actually 2 threads per millimeter. Whereas the M8-1.25 uses 1.25 millimeters of length to complete one full thread which would convert to .8 threads per millimeter. Personally I prefer the imperial system.