# Line

## Line

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

• If A and B are two points in the plane, the line that passes through A and B contains all the points in the plane that are aligned with A and B.
• A line can also be called a curve with a constant zero curvature

### 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.