Horizontal is an orientation relating to, or in parallel with the horizon, and the opposite of vertical. A horizontal line goes from left to right (or vice versa) where a vertical line goes from top to bottom.
In Cartesian coordinates of the form (x,y), the horizontal axis is conventionally used for the x coordinate, with positive numbers extending to the right side of the vertical axis. The equation y=c, where c a constant, represents a horizontal line. In an complex number, the horizontal axis is used for the real component.
In perspective drawing, a set of parallel horizontal lines which are not parallel to the horizon will, if extended, meet at a single point on the horizon known as the vanishing point. This is a common feature of pictures of straight roads or railway lines.
In construction, a spirit level helps creating horizontal structures.
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