A measuring gauge designed to indicate the wear (stretch) in bicycle chain. Typically a chain has to be replaced at the stretch of 0.75%. If it is done at this point in most cases the old cassette can be saved, whereas a chain replaced at 1mm (1%) stretch will normally require a new cassette/freewheel. The lifespan of a chain depends on several factors such as riding conditions (weather), riding style (crossing the chain, pedalling in one or two gears only etc.), chain maintenance, riding terrain etc.
