Horizontal lines run left to right, parallel to the horizon, with slope 0 and equation y = k. Vertical lines run top to bottom, perpendicular to the horizon, with undefined slope and equation x = k.

What is the difference between a horizontal and vertical line? Learn how to identify vertical, horizontal, parallel and perpendicular lines.

This Maths article shows how the mirror line can help you to recognise vertical lines of symmetry on shapes and letters.