A classical Greek problem is to trisect an angle using ruler and compasses only and this is impossible. However this can be done using a carpenter's square. To trisect the angle ABC, first draw the line DE parallel to BC and mark points P, Q and R such that PQ = QR = ST. Place the square so that:

• R is on the line DE
• P is on the line AB
• The edge QT contains the point B.

Prove that PBQ = QBR = RBC.

For more about angle trisection see the St Andrew's University History of Maths website