Set squares are used for drawing perpendicular and parallel lines, according to Mathsteacher.com. The two set squares are named according to their angles. One has 90, 30 and 60 degree angles and the other has 90 and 45 degree angles.
Set squares are often used in combination with a T-square to draw more accurate angles. Technology Student states that set squares can only be accurate when they are used in combination with T-squares. Set squares are used while resting on top of a T-square which is pushed up against the edge of a board.
A common error that occurs when using set squares and T-squares is allowing a gap to develop between the edge of the board and the T-square.