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

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