Geometry for Elementary School/Our tools: Ruler and compass

Introduction

This is a picture of a pair of compasses and a ruler.

How to draw a line

Using a ruler (based on Book I, Postulate 1), we can draw a segment of a straight line.

We will use the notation ${\overline {AB}}$ for the line segment the starts at A and ends at B. Note that we don't care about the segment direction and therefore ${\overline {AB}}$ is the same as ${\overline {BA}}$.

Line segment ${\overline {AB}}$

Line segment ${\overline {BA}}$

How to draw a circle

• Using the compass (based on Book I, Postulate 3).

We use the notation $\circ A,{\overline {AB}}$ for the circle whose centre is the point A and its radius length equals that of the segment ${\overline {AB}}$.

Note that in other sources, such as Euclid's Elements, a circle is described by any 3 points on its circumference, ABC.

The center-radius notation was chosen because of its suitability for constructing circles with a ruler and a compass.