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It's basically a bracket that goes onto your rear sprocket then a long thin rod as a reference line to see if your chain is properly aligned. Use on left or right side. An improperly aligned chain and sprocket set will cause both of them premature worn out.
When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.