Straightedge and Compass
- Ku, Yin Bon (Albert)
Straightedge can be regarded as a ruler without any marking. The following are exactly what a straightedge can do:
- Draw a unique straight line through two distinct points
- Extend a line segment arbitrarily far in both direction
Compass is a V-shaped drawing tool with a sharp point on one arm and a pencil on another arm. The two arms are joined by a hinge so that the openings of the arms is adjustible. Obviously, a compass is mainly used for constructing a circle centered at any given point with any given radius. There are two types of compasses:
- Modern compass - We can keep the opening fixed when the compass leaves the plane and carry to another location for construction.
- Euclidean (or collapsible) compass - The compass "forgets" the width of the opening when the compass leaves the plane i.e. we cannot keep the opening fixed.
We can use the following applet to simulate geometric constructions using straightedge and compass.