Gears

Various gears
Gear wheels come in various materials, diameters and tooth sizes.
One tooth is also called a Cog and the proper name for the wheel is a Cog Wheel or a Gear Wheel.
The Pitch Circle Diameter (PCD) of a gear wheel is the diameter of the circle where the teeth on meshing wheels touch.
Imperial tooth sizes are specified by the number of teeth per inch of PCD. This is called the Diametrical Pitch (DP).
Gear wheels with the same Diametrical Pitch will mesh together.
Metric sizes are specified by the PCD divided by the number of teeth. This is called the Module number.
So a 1cm (centimetre) PCD gear wheel with 20 teeth will have a Module number of 0.5 and be called a 0.5 module or 0.5 M gear wheel
A 5cm diameter gear wheel with 50 teeth will have a module number of 1.0 and be called a 1 module or 1 M gear wheel
Gear wheels with the same Module number will mesh together.

On the left are some black nylon 1M gear wheels - from MFA.
At the top there is a white worm-wheel which matches the 1M gear wheels.
In the centre are some brass 0.5M gear wheels - from Hobbies.
Below some black nylon 28 DP gear wheels and a rack - from some hobby robot web shops in the UK.
On the right a pack of worms and wheels used in model trains - from model shops.
NOTE in a proper Worm-wheel and Cog-wheel set the matching Cog-wheel has the teeth at an angle to match the helix angle of the tooth on the Worm-wheel. For light duty hobby use the Worm-wheel tooth has a modified shape to work with ordinary gear wheels.