How to tell the two set squares apart
One triangle has two equal sides and angles of 45°, 45° and 90°. The other has three unequal sides and angles of 30°, 60° and 90°. The two sides that form the right angle are the legs. The longest side, opposite the right angle, is the hypotenuse.
How to position your set squares
Draw a sloping line to use as your reference. Line up the hypotenuse of the 45° triangle with it. Bring the 30°–60° triangle alongside until its long edge rests against one leg of the 45° triangle. Hold the 30°–60° triangle steady with your non-writing hand. Both tools should lie flat, with their edges touching along a length rather than just at one corner.
Draw a parallel through a point
Mark a point off the reference line. Keeping the guide still, slide the 45° triangle along it until its hypotenuse passes through the point. Draw lightly along that edge. It has moved without turning, so the line you have drawn runs in the same direction as the reference line. You can use the same movement to add further parallels.
Switch legs to draw a perpendicular
Return the 45° triangle to its starting position and keep the guide still. Rotate the 45° triangle a quarter turn on the paper so that its other leg now rests against the same edge of the guide. Its hypotenuse has turned through 90°. Slide it until the hypotenuse passes through your chosen point, then draw along it. The point can be on the reference line or off it.
Check the angle at the crossing
Place the 90° corner of either triangle where the two lines cross. Each leg should line up with one of the lines. If a gap gradually opens up, check whether the guide shifted when you turned the other triangle. For practice, draw three parallels and cross all three with a perpendicular. You can check each crossing the same way.
To rule lines for writing on a card, use a graduated ruler to mark equal intervals along the perpendicular. Draw one parallel through each mark to give the rows even spacing.
A useful place to practise is the back of a postcard, where three parallel lines can guide the address.
Technical drawing materials from the Xunta de Galicia