Points, lines, rays and segments

1Four figures with names

• A point marks one exact place. We draw it as a dot and name it with a capital letter, like point P.
• A line is perfectly straight and goes on forever in both directions. We draw arrows at both ends. We name it with two of its points: line AB.
• A ray starts at one point and goes on forever in one direction, like the light from a torch. We name it from its starting point: ray CD starts at C.
• A line segment is a piece of a line with two endpoints. It stops at both ends, so we can measure it. Segment EF goes from E to F.

A line and a ray go on forever, so we cannot measure how long they are. Only a line segment has a length.

2Measuring a line segment

Put the 0 mark of your ruler exactly on one endpoint. Keep the ruler's edge along the segment. Read the number at the other endpoint.

Segment MN starts at 0 and ends halfway between 6 and 7. The small marks are millimetres (mm), and 1 cm = 10 mm. N is 5 small marks past 6, so MN = 6 cm 5 mm, which is 65 mm.

3When the ruler does not start at 0

If a pencil lies from the 2 cm mark to the 9 cm mark, it is not 9 cm long. Subtract: 9 − 2 = 7 cm. The pencil covers 7 whole centimetres.

4Drawing a segment of a given length

To draw a segment 8 cm long, mark a point at 0, then a point at 8 on your ruler. Join the two points with a sharp pencil.

Try it yourself

Work each one in your exercise book first. Then tap Show answer to check.