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.

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).

Math Open Reference now has a Common Core alignment.

See which resources are available on this site for each element of the Common Core standards.