A continuous arrangement of an infinite number of points aligned on a plane.

### Notation

Although there is no universally recognized notation for a line, it is generally denoted by a lowercase letter or by two of its points (expressed in capital letters).

Therefore, the line that passes through points *A* and *B* is written as \(\overline{AB}\). It is also represented by the letter *l*.

### Properties

**Invariant line**

A line is invariant if it remains unchanged under a geometric transformation.**Boundary line**

A line that divides a geometric plane or a Cartesian plane into two half plane.**Oblique line**

A line that crosses another line without being perpendicular to the other line.**Directed line**

Line on which there is a positive and negative direction.**Special Line (or segment) in a triangle**

Name given to each angle bisector of a triangle, the perpendicular bisector of each of its sides, the median to each of its sides or each of its heights.