Measure Chain Wear

"One measurement is worth a thousand expert opinions"

Chain and cogs wear in relation to one another. Peculiar to any given rider, at some point the chain will no longer properly engage the cog, resulting in sudden loss of drive or skipping under heavy exertion. At this point, the only effective remedy is replacement of chain, cassette or freewheel, and possibly chainrings (front cog/s). 

With modern drive parts costing what they do, several very good tools can indicate wear points when chain-only replacement is sufficient. Caught in time, the rest of the system may last through 3 or 4 chains.