Lines and Line Segments (continued)
The picture below shows the Lunar Laser at the McDonald Observatory in Texas. The laser beam is used to measure distances to the moon.
If the laser beam were able to go on forever in both directions, it would be an example of a line. However, the line has a start point at the observatory and ends when it hits the moon. Since it has endpoints, the laser beam is actually an example of a line segment.
Any portion or internal part of a line is called a line segment. For example, in the figure below, MN is a line segment from the line AB.
In other words, a line segment is a line that is limited at both end points and its length can be measured. A line segment is usually called just a segment and is denoted by the labels of the endpoints with a sign "-" above. For example, the segment in the figure above is denoted 
.
If points lie in a straight line, we say that they are collinear.
Points that do not lie in a straight line are called non-collinear.
Can you name some other line segments contained in line AB above?
, letters can be written in the opposite order.