Lee Valley Miter Hook
Few things are as frustrating as attempting to measure accurately from an inside corner. This miter hook, developed by a trim carpenter, makes it simple. Just press the pins into the end grain (the pins are slightly angled to draw the hook in snugly) and hook your tape measure into the slot that is exactly even with the miter end. A rare-earth magnet grips the tape as you measure.
Also usable for outside corners where the hook works the same way, except pins are not necessary. The miter hook has 1/8" and 1/4" wide ledges – perfect for accurately setting reveals in house trim. The hook is handy in a shop as well – anywhere you need to measure mitered corners.
Injection molded using a high-impact plastic, it has hardened pins and two rare-earth magnets. It is fully reversible; the magnets are exposed on both sides.