This is the most accurate chain wear indicator money can buy and it is dead simple to use. Simply hook the rounded left tab onto any inner roller of the chain and then measure elongation with the tab on the right side of the tool. The KMC Chain Checker will measures up to 2.5mm of elongation and fits all chains regardless of width. Digital read out is precise and it's almost impossible not to get a perfect read out every time. This tool will let you know when you chain should be replaced and help reduce unnecessary wear on other expensive drive train components.