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


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.


  • 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.

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