Online Tips Index / Assembly / 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, measuing long diagonals accurately can be difficult. To make this job easier, I made a pair of simple tape holders.
Each holder is just a scrap of ¾″ thick plywood cut to 4½″ square. Two pieces of ¼″ Masonite 2″ × 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.