Geometry for Elementary School/Our tools: Ruler and compass

Geometry for Elementary School
Construction Our Tools: Ruler and compasses Constructing equilateral triangle


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   for the line segment the starts at A and ends at B. Note that we don't care about the segment direction and therefore   is the same as  .

Line segment  
Line segment  

How to draw a circle


We use the notation   for the circle whose centre is the point A and its radius length equals that of the segment  .


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.