In geometry, a horizontal line is one which runs from left to right across the page. It comes from the word 'horizon', in the sense that horizontal lines are parallel to the horizon.

The horizon is horizontal

Its cousin is the vertical line which runs up and down the page. A vertical line is perpendicular to a horizontal line. (See perpendicular lines).

Coordinate geometry

Other line topics

• Point definition
• Line definition
• Vertical lines
• Horizontal lines
• Line segment
• Midpoint of a line
• Ray
• Angle
• Opposite rays
• Intersection
• Parallel lines
• Transversal
• Line bisector
• Perpendicular bisector
• Coplanar
• Collinear points
• Plane