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Common Core Standard  G.CO.1
green lines geometry
Collinear is when you have three or more points on a straight line. Points A,B, and C are collinear.
C
B
A
point on a line
point on a line
point on a line
collinear line
line segment AC
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.

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.

How do you name a line?

line 1 and line 2 crossing in an x
line 1 and line 2 crossing in an x

Names of Lines and Rays




In Geometry a line has these properties:  
  • One-dimensional 
  • Has length
  •  No width 
  • No height,
  •  No curves 
  •  Extends in both directions infinitely





In order to name a line or a line segment you have to first name a point. In this example you would have points A, B, and C.

A capital letter is used when naming a point.

Step 1. Pick two points

Step 2. Use Capital letters

Step 3. At this point you can label a line by drawing an arrow over the capital letters, or draw a straight line for a line segment
line segments AC or CA  BC or CB
Line 2
Line 1
Line2
Line 1

Line 1  Line q
Line 2  Line m
two lines crossing
A
C
m
q
B
q
C
B
A
m
Line 1
Line2

What is a ray in Geometry

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
ray in geometry
ray in geometry with endpoint
ray D
ray AB
A
B
D
Endpoint
Name of Ray =
Name of Ray =
up arrow
Math has it's own unique language and method for communicating, and here are some tips on how lines are expressed.
Line Segment 1
Line Segment 2