The metric measurement always begins with an M, (which stands for metric).
The first number is the major diameter of male threads.
The second number is the pitch. Pitch is is the length of one complete thread.
This will then usually be followed by the length of the threaded item (this measurement will occasionally be given in inches, even on the metric scale).
For a certain range of diameters, there is a set pitch which allows the measurements to be written simply as M#. The pitches for these are standard coarse measurements set out by the international standards organisation (ISO). The chart is an example of these.
