A four sided object with opposite sides of equal lengths will have square corners if the length of the diagionals are equal.

3 - 4 - 5 Rule

A four sided object with opposite sides of equal lengths will have square corners if the length of the diagionals are equal.

A triangle with sides measuring 3 units X 4 units X 5 units ( or any multiple of them such as 6 - 8 - 10 or 12 - 16 - 20 ) will always have a square corner where the 3 and 4 unit sides meet. To square a corner measure 3 units along one side and 4 units along the other side, adjust the sides until the diagional equals 5 units.

Note: If you are making several articles the same tack guide strips to the plywood or 1 X 4's for placement of material.

Corner Jigs for Boxes

Use lengths of angle iron to keep corners of a box or carcass square, clamp the angle iron so it is just snug, then wrap one or two web clamps around the outside of the box. This will work for any type of joint, usually two at opposite corners will be sufficient, clamp both ends of angle iron if possible. Round the outside corner of the angle iron with a file to make room for any glue that squeezes out.

Cut triangles from 3/4" plywood or MDF for narrow boxes. Round the inside corner for glue squeeze out on these as well.

Clamp-It® Assembly Square and Clamps
Align it, clamp it, and fasten it to get perfect 90 degree angles every time! Here's a solution that's so simple, you'll wonder why it took so long to develop! Clamp-It® Assembly Square and Clamps

Squaring a Board

To square a board follow these steps:

Run board through jointer or hand plane one edge and one face straight.

Run board through planer to attain desired thickness, with straight face down.

Run board through table saw and cut second edge with guide set aprox. 1/16" wider than finished size of board.

Run board through jointer or hand plane second edge to desired width.

Cut one end of board square aprox. 1/8" from end.

Measure length and cut second end.

For panels glue up boards before trimming ends.

• Cross cutting on a table saw is easier if a cut-off sled is used.
• For exceptionally curved boards true one side with this jig before jointing.

To Test a Square

Not all squares are created equal, when purchasing a square check it for accuracy, and occasionally afterwards, dropping or banging it against something can throw it out. When checking a framing square check both the inside and outside for accuracy. There is a link here about checking and adjusting squares.