Canson Set Square, 60 Degrees, Clear, 27 cm

A set square is a fixed-angle triangle. This one carries the 30, 60 and 90 degree angles, and between it, a 45 degree square and a straight edge you can construct almost every angle a technical drawing needs without reaching for a protractor.

Use it against a T-square or a parallel rule rather than freehand: the long edge rides the straight edge, the square gives you the vertical or the angle, and that is how a drawing ends up with genuinely parallel lines. Clear acrylic lets you see the drawing underneath while you work. The thing that ruins a set square is ink running under the edge, so keep a bevelled edge towards the pen or lift the square clear when you ink. At 27 cm this is a drawing-board tool, not a pencil-case one.

• 30, 60 and 90 degree angles
• Clear acrylic — see the drawing underneath
• 27 cm, drawing-board size
• Use against a T-square or parallel rule for true parallels