The shop-built router jig detailed in this video uses an innovative system to position a workpiece square and centered when cutting matching mortises for loose mortise-and-tenon joinery. Designed by Indiana woodworker David Lehman, the jig relies on two vertically aligned, rotating circles with protruding metal bolts to align a workpiece, and a moveable fence to set the width of a mortise.

In this video, Fine Woodworking editor Charlie Reina puts Lehman's circle jig to work cutting matching mortises. Reina demonstrates how to set up the jig to cut both mortises and offers helpful tips for this fast and efficient joinery technique.