Crucible Pinch Rods

While there are many ways to check whether an assembled carcase is square, the easiest is to use pinch rods to compare the diagonals. Unlike measuring the diagonals, pinch rods do not depend on reading and remembering dimensions, so they are much less prone to error. Pinch rods also let you bring a carcase back to square using visual cues:

Here's how:
Set the pinch rods to the shorter diagonal of the carcase
Place the pinch rods in the longer diagonal
Apply clamp pressure across the long diagonal until half of the gap between the tool and the carcase disappears

Pinch rods are also handy for transferring inside dimensions of a carcase, say from a face frame to a door, and for transferring dimensions from full-size drawings to the actual piece.

The Crucible pinch rods are based on a vintage example seen at Roy Underhill's. The bodies and thumbscrew are milled from brass in Kentucky and include two No. 6 screws to attach the hardware to the wood of your choice. The custom-milled thumbscrew applies pressure at a concentrated point, making a small indent in the wood that locks the setting even if you drop the tool. If the wood ever becomes too worn, you can easily replace it.

Facts:
Brass bodies and thumbscrew, two No. 6 screws
Sticks not included: two straight, clear, dry sticks 3/8" × 3/4"
With 32" sticks it handles assemblies from 32" up to about 58", enough for most drawers and casework
Made in the USA