There are two main chain sizes, 1/2 x 3/32 and 1/2 x 1/8. The 1/2 refers to the distance from pin to pin. The 3/32 and 1/8 refer to the width of the space between the inside sideplates. 1/2 x 3/32 chains are referred to as "derailleur chains" because they're narrow chains for use with bicycles with derailleur gearing. Note: When buying this type of chain be sure to get the one made for the number of cogs on your rear wheel, such as 8-, 9- or 10-speed. The other size chains, 1/2 x 1/8 are referred to as "coaster-brake" or "BMX chains" because they're wider, and for use with coaster-brake, BMX and certain 3-speed drivetrains. If you have any questions about chain compatibility, please contact us.