How to Construct a Triangle with a Compass and Straightedge

High school Design and Technology students are often required to have the technical drawing skill to construct basic geometric shapes. This article explains how to construct triangles using only a compass and straightedge (such as set square or ruler).

Students should be prepared to construct equilateral triangles (all sides same length), isosceles triangles (two sides of equal length), and scalene triangles (no equal sides).

How to construct a triangle when given three sides

• Draw the first side of the triangle AB;
• Scribe arcs of radius of each side length from each end;
• Draw the final two sides of the triangle to point C, where the two arcs intersect.
• If constructing an isosceles triangle, both arcs will be the same radius.

How to draw an equilateral triangle when given one side

• Draw side AB;
• Scribe arcs from each end of the line, radius = side length;
• Draw the final two sides of the triangle to the point where the arcs intersect.