Horizontal/ˌhɔːrɪˈzɑːntl/ Definition 1 of 4Pro Tip 1/3Visual Imagery Imagine a flat surface or line running left to right, like lying down. She drew a horizontal line on the paper. Pro Tip 2/3Not Vertical Remember that 'horizontal' is the opposite of 'vertical', which goes up and down. The shelves were installed horizontally, not vertically. Pro Tip 3/3Aligned with Horizon Think of lying back to gaze at the sky; the line where earth meets sky is horizontal. At sunset, the horizontal glow was breathtaking.