Newbie Question: What do speeds mean on chains? I'm shopping for a new chain, and I noticed some inconsistencies when discussing speeds. Specifically, speeds applied to the cassette refer to how many layer of gears it has (I presume). Speeds, when applied to bikes, refers to the total number of speeds it has with the cassette (or freewheel) multiplied by the chainring on the crankset.

Soooo... what do the speeds listed on a chain refer to (ignoring the compatibility issues -- those are very well documented)?

For example, if I have a cassette with 5 gears and a crankset with 2 gears, do I need a 10 speed chain or a 5 speed chain?
A bike with a 5 cog cassette requires a 3/32" 5 speed chain which has also been referred to as the "10 speed chain". This is different to a modern 10 speed chain though as below.

In rrecent times speeds refer to the number of cogs that make up the rear cassette. Outside of big box stores gearing is generally specified as YxZ. So in a department store you may see a bike advertised as having 21, 24, 27, 20 or 11 speeds whereas in a bike store it would be sold as 3x7, 3x8, 3x9, 2x10 or 1x11 (right up to 1x12 which strangely is currently referred to just as Eagle!)

The reasoning behind specific chains for different speeds is as they have increased the number of gears available on a bike they have maintained the same freehub width over the last 4 iterations. 8, 9, 10 and some 11 speed cassettes all work on the same width hub. This has been achieved by spacing the cogs closer together and producing narrower chains.

There is generally tolerance in a chain to go up/down 1 speed without issue. Ie. an 11 speed chain can be used on a 10 speed cassette.