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Horizontal
A horizontal line is one which runs left-to-right across the page.

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

A line will be horizontal if any two points on the line have the same y-coordinate.
See Horizontal line (coordinate geometry).
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