How to Use a Geometry Compass

A geometry compass has two legs joined at a hinge: a needle point that stays fixed, and a pencil point that traces the circle. The distance between them, kept constant as you rotate, is the radius.

Basic steps

• Decide where the center of your circle should be, and place the needle point there.
• Open the compass to your desired radius. On paper, measure this against a ruler; on the online compass, drag outward or type an exact radius.
• Keeping the needle point still, rotate the pencil leg a full turn around it.
• To draw another circle from a different center, move the whole compass and repeat.

Common mistakes

The needle point slipping mid-rotation is the most common problem on paper, producing an oval instead of a circle. The digital equivalent doesn't have this issue, since the center is fixed by a click rather than physical pressure, which is one advantage of practicing constructions on screen first.

Beyond circles: basic constructions

A compass is also used for constructions that don't look like a simple circle. To find the perpendicular bisector of a line segment, open the compass wider than half the segment's length, draw an arc from each endpoint, and connect the two points where the arcs cross. To copy a length from one part of a drawing to another, set the radius once at the original length and don't adjust it until you've placed the copy.

Practice constructions on the online geometry compass, or measure real objects first with the online ruler.