From DT Online
Squareness refers to things being a right angles or 90 degrees to each other (i.e. as the corners of a Square). Checking that something is Square is a common designing and making requirement and operations often have to be performed at right angles to an edge or surface (e.g. marking a line, cutting material to length, drilling a hole).
Carpenters and Engineers use a Try Square to draw or scribe lines at right angles to an edge or to test if a corner is Square. They may also use a Framing Square to check that corners of a larger construction are Square - but this can also be done by testing that its Diagonals are of equal length (since this is a characteristic of all squares and rectangles).
Note: The Diagonals of an Isosceles Trapezium are also equal so checking their length would also test if such a regular shape was ‘squared up’
In Technical Drawing a Draughtsman would use a T-square pressed against the edge of a drawing board to draw parallel lines across the paper and place Set Squares to draw lines at 90deg, 45deg, 60deg, and 30deg to it.