Like the hook on the end of a retractable tape measure, the one on the end of this steel rule makes it easy to measure from an overhanging edge.
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The hook can be repositioned along the end of the rule to work with thin materials; to reach over a routed edge, as shown; or you can remove it completely if you like. Four scales (one on each edge, front and back) let you choose incremental markings from 1/8" to 1/64", depending on how accurate you want to be.