Width

Chains come in ³⁄₃₂ in (2.4 mm), ¹⁄₈ in (3.2 mm), ⁵⁄₃₂ in (4.0 mm), or ³⁄₁₆ in (4.8 mm) roller widths, the internal width between the inner plates. ¹⁄₈ in (3.2 mm) chains are typically used on bikes with a single rear sprocket: those with coaster brakes, hub gears, fixed gears such as track bicycles, or BMX bikes. Chains with ³⁄₃₂ in (2.4 mm) wide rollers are generally used on bikes with derailleurs such as racing, touring, and mountain bikes.^[17] Fixed sprockets and freewheels are also available in ³⁄₃₂ in (2.4 mm) widths so fixed-gear and single-speed bikes can be set up to use the narrower and lighter ³⁄₃₂ in (2.4 mm) chains. Finally, chains with ⁵⁄₃₂ in (4.0 mm) wide rollers are used on freight bicycles and tricycles.

With derailleur equipped bicycles, the external width of the chain (measured at the connecting rivet) also matters, because chains must not be too wide for the cogset or they will rub on the next larger sprocket, or too narrow that they might fall between two sprockets. Chains can also be identified by the number of rear sprockets they can support, anywhere from 3 to 12, and the list below enables measuring a chain of unknown origin to determine its suitability.

• 6 speed – 7.3 mm (⁹⁄₃₂ in) (Shimano HG), 7.1 mm (⁹⁄₃₂ in) (SRAM, Shimano IG)
• 7 speed – 7.3 mm (⁹⁄₃₂ in) (Shimano HG), 7.1 mm (⁹⁄₃₂ in) (SRAM, Shimano IG)
• 8 speed – 7.3 mm (⁹⁄₃₂ in) (Shimano HG), 7.1 mm (⁹⁄₃₂ in) (SRAM, Shimano IG)
• 9 speed – 6.5 to 7.0 mm (¹⁄₄ to ⁹⁄₃₂ in) (all brands)
• 10 speed – 6.0 to 7.0 mm (¹⁄₄ to ⁹⁄₃₂ in) (Shimano, Campagnolo)
• 10 speed (Narrow) – 5.88 mm (⁷⁄₃₂ in) (Campagnolo, KMC)
• 10 speed (Narrow, Direction) – 5.88 mm (⁷⁄₃₂ in) (Shimano CN-5700, CN-6700, CN-7900)
• 11 speed – 5.5 to 5.62 mm (⁷⁄₃₂ to ⁷⁄₃₂ in) (Campagnolo, KMC, Shimano CN-9000)
• 12 speed – 5.3 mm (¹³⁄₆₄ in) (SRAM)

The Wikibook, “Bicycle Maintenance and Repair”, has more details on this topic. Shimano uses the same chain types on 6, 7,and 8 speed designs.

https://en.wikipedia.org/wiki/Bicycle_chain