A motorcycle chain is the most common method used to connect and drive the rear wheel in a motorcycle. The others are belt drive and shaft drive.

Chain size is in inches, the three digit number used to discribe the chain size is based on the "rule of eights". The first digit is for chain pitch. This is the distance between the pins. The second and third digits combine for roller width, with a decimal point inserted between the second and third digits.

For example, a 420 chain has a pitch of 4/8" = 1/2" (12.70mm) and a roller width of 2.0/8" = 1/4" (6.35mm).

A 525 chain (the size my bike uses) has a pitch of 5/8" (15.88mm) and a roller width of 2.5/8" (7.94mm).

A 530 chain has a pitch of 5/8" (15.88mm) and a roller width of 3.0/8" (9.52mm).

There are a number of chain sizes in use, and chain manufacturers may have slightly different specs for roller diameter, sprocket width, etc. But a 525 chain from one manufacturer will have the same pitch as a 520, 525, 530, or 532 chain from another and run smoothly on any 525 sprocket from any manufacturer.