Which way is horizontal?

A horizontal line runs parallel to the horizon, at a 90 degree angle to a vertical line. A person lying down in a flat position would create a horizontal line; a standing person would create a vertical line.

The word "horizontal" is usually used as an adjective. It may be used to refer to something that is moved across a plane in a left-to-right motion, just as rooks are moved horizontally across a chess board. It is also used to show that something is the same level as another. For example, an employee could receive a horizontal transfer within their company, meaning that the position he is moving to is at the same level as the one he is leaving.