GeoGebra Classroom

# Straightedge and Compass

## Straightedge

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
Notice that it cannot be used for measuring length.

## Compass

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.

## Geometric Constructions

We can use the following applet to simulate geometric constructions using straightedge and compass.