This is the same tool that we use to narrow a rear end. It makes measuring axle tube lengths easier. It bolts to the rear end and provides a slot to hold the end of the measuring tape. It comes predrilled with holes that match the rear end and locate the center line slot correctly. This tool fits Ford 9", Dana 60, Chevy 12 bolt, Mopar 8-3/4" and Early Olds rear end housings.