Fig 1Anatomy of a gear20° full-depth tootheverything scales with m
Pitch circleThree teeth of a 24-tooth gear, drawn huge. Everything on a standard gear is a multiple of one number, the module m, in millimetres. The pitch circle is the imaginary rolling circle, with diameter m·z. Teeth stick out one module above it (the addendum) and go 1.25 modules below it (the dedendum). The extra quarter is clearance for the other gear's tip. Two gears mesh only if their module and pressure angle match.