A horizontal line is a straight line that goes from left to right, which can be open or closed in form. On a coordinate plane, a horizontal line runs parallel to the x-axis.
A coordinate plan has an x-axis and y-axis. All points on a horizontal line have the same value for the y coordinate in their ordered pair, (x,y). The opposite is true for a vertical line. A vertical line runs parallel to the y-axis, and points on the line contain the same value for their x coordinates. The slope of a horizontal line is zero, meaning the line does not rise or fall.