The Square
Behind the panel The Square · series D, Tools Through Time.
Buildings are made of right angles. If the first corner of a slab is out of square, every wall, floor sheet and roof sheet above it will fight the error.
The rule of 3-4-5
Builders knew long before Pythagoras was credited with the theorem that a triangle with sides in the ratio 3:4:5 has a right angle. Measure 3 units along one line, 4 along the other; if the diagonal is 5, the corner is square. Multiples work too: 6-8-10, 9-12-15. Carpenters also check whole rectangles by measuring both diagonals: when they match, the frame is square.
The try square and the framing square
The try square, a blade fixed to a thicker stock, marks a line square across a board. The framing (steel or carpenter's) square is a large L of flat steel, 24 by 16 inches. In 1819 Silas Hawes of South Shaftsbury, Vermont, patented a one-piece steel square; later squares carried stamped tables for rafter lengths, board feet and brace lengths, making the square a slide rule for carpenters.
The speed square
In 1925 Albert J. Swanson introduced a triangular square with a lip that hooks the edge of a board and a pivot point with angle marks. It marks square and 45-degree cuts and rafter angles quickly, and it is now standard in framers' belts.
Square at scale
On large buildings, square is set out by surveyors with total stations using coordinates, not triangles. But every floor still ends with a crew checking diagonals, because a tape measure is fast and it does not lie.
