Originally Posted by ivan_yulaev
You need to measure wear on your chain. Align the 1" mark with the middle of one rivet, the 12" mark should fall in the middle of another rivet (exactly). If you have less than 1/16 of margin, you're fine. 1/16" of stretch is when you should change the chain. If your chain stretches to 1/8" longer over 12" or more, you will have to replace the cassette. Otherwise, the cassette should last many chains...
Actually, you'll need to align the 0" mark with the middle of one rivet, and the 12" mark should fall in the middle of another rivet. If you use the 1" mark, you'll only be measuring stretch across 11" on your ruler, and so you will not accurately detect 1/16" of stretch (in 12") until it's gone too far. In other words, measuring from 1" to 12" on your ruler will fool you into thinking the chain is fine, when in fact it could be stretched too much.