Checking Diagonals

Comparing the diagonals of a cabinet or a box is a great way to check it for square. But when you're working alone in the shop, measuring long diagonals accurately can be difficult.
Each holder is just a scrap of ¾" thick plywood cut to 4½" square. Two pieces of ¼" Masonite 2" x 4½" are screwed to two edges of the plywood that make up a corner. Then, to hold the tape and allow for an accurate reading, the corner is mitered to form a slot that fits the blade.