What makes a mod1 pinion mod 1?

[mikerobinson83]

Hi i am trying to find out as much about pinions as possable as i am looking to design some of my own (i am a mechanical engineering student so i have access to the machines needed to make a couple of prototypes) i am going to start with mod 1 pinions as this is what i use in my converted lightning 10.

i know it has something to do with the tooth sizes but am unsure what in partcular i am looking at.

Thanks for all your help Mike