How do you check an opening is square?

Framing
Layout

A framing square only tells you about the corner it is sitting in, which is why a rough opening can pass four corner checks and still be out of square. Two diagonals with the T1R Range settle it in one comparison, and on anything larger than a door that is faster than squaring each corner. Both readings stay on the device for the record.

Why diagonals beat a framing square

A square only reads the corner it sits in
Matching diagonals mean it is square
T1R takes both corner to corner
Faster than squaring each corner
Both readings stay on the device

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Measure both diagonals and compare them. In this framed opening the T1R Range takes corner to corner one way, then corner to corner the other. If the two numbers match, the opening is square. On anything bigger than a door that is faster than a framing square, and both readings stay on the device.