Collinear is when you have three or more points on a straight line. Points A,B, and C are collinear.

C

B

A

Technically a line is a group of points on a straight line. A line segment is different than a line because it has endpoints compared to a line that does not have endpoints, and extends in both directions infinitely. Check out the video to the left for a visual picture of a line.

A point marks an exact location. In Geometry class we usually mark a point by making a dot with our pencil, but in reality a point has no size. The point does not have any size or dimensions because it marks a place, and is not an object.

Points are named using an upper case letter.

You also can use lower case letters to name a line. For example:

Once you have points labeled it makes naming a line simple.

A ray is a line with a starting point called an endpoint. A ray travels in one direction for infinity and can pass through other points along the way to infinity.

Properties of a Ray in Geometry

Has a starting point called the endpoint

Has length

No height

No width

Extends in one direction infinitly

Sometimes called a "half line"

Naming a Ray in Geometry

Start at the endpoint

Use an arrow for the direction of the ray

If it passes through an endpoint it is included in it's name

A

B

D

Endpoint

Name of Ray =

Name of Ray =

Math has it's own unique language and method for communicating, and here are some tips on how lines are expressed.