There are two representations of straight lines: one is represented by two uppercase English letters, which is pronounced as straight line AB or BA; Second, expressed in lowercase letters, the straight line AB can also be written as a straight line L.

The ray is represented by two capital letters, the first letter represents the endpoint and the second letter represents any point on the ray. The alphabetical order cannot be reversed. Ray OA cannot be recorded as ray AO.

A line segment is represented by letters or lowercase letters representing its two endpoints. Sometimes these letters also indicate the length of a line segment, which is marked as line segment AB or line segment BA and line segment A. A line segment can extend infinitely in two directions, namely, extending line segment AB or extending line segment BA in the opposite direction.