Lines, Line Segments and Rays

by on May 17, 2011

So, you think you know what a line is.  Well, your 2nd or 3rd grader just might challenge your definition.  Lines, line segments, and rays are all second and third grade math concepts, and although they are similar, they are not the same. What exactly do these geometry terms mean?  Read on to find out.

What is it?

A line is defined as an infinite set of points forming a straight path, extending in both directions.  It does not have a beginning or an end.

A line segment is a part of a line, defined by two endpoints.

A ray is a part of line that has only one endpoint, and extends indefinitely in one direction.

NAMING LINES, LINE SEGMENTS AND RAYS

We typically name a line, line segment or ray using uppercase letters.  For example:

Line AB

Line Segment EF

Ray GH

What’s the Difference?

You may have noticed that a line has arrows on both ends.  This is to indicate that the line goes on forever in both directions.  There are 2 points on the line that are labelled with uppercase letters.

A line segment has an endpoint on both ends, indicating that there is a distinct beginning and end.  Each endpoint is labelled with an uppercase letter.

A ray has one endpoint, and an arrow on the other end.  It too is labelled with uppercase letters on each end.

Tips/Activities

• As your child learns about lines, line segments and rays, he will also be introduced to third and second grade math skill vocabulary words such as intersect and parallel .
• Intersecting lines are lines that cross one another.
• Parallel lines are two lines that are equal distance from one another (if the two lines were extended infinitely, they would never cross.
• Your child will likely be asked to draw lines, line segments and rays at school.  A typical question would look like this:

Draw —>AB that intersects with <——-> CD.   This would read: “Draw ray AB that intersects with line CD”.  Your child would draw:

A——-B>   with <C———–D> CROSSING ray AB. (The drawing should form a cross shape.)

What To Watch For

• As your child is naming lines and line segments, be sure he notices that they can be read in either direction.  Using the examples from the Naming Lines, Lines Segments and Rays above, the line could read either “line AB” or “line BA”.  The line segment could read either “line segment EF” or “line segment FE.”
• The ray is the only one that doesn’t follow this rule.  A ray must be read in the direction of the arrow.  So, using the example   G.——–.H>,  this ray would be read “ray GH”, and not “ray HG”.  This is one of the most common mistakes students make.

Want More?

• As always, playing math games at home is a great way to reinforce math skills learned in school.