Horizontal means lying flat or level, going straight across from side to side rather than up and down. When you lie down in bed, your body is horizontal. The floor beneath your feet is horizontal. The horizon, where the sky seems to meet the earth, gives us the word: it's that perfectly level line stretching across your view.

Think of a ladder into an attic. The rungs you step on are horizontal, going across from one side to the other. The sides of the ladder going up and down are vertical, which is the opposite of horizontal. A horizontal line looks like this: _____. A vertical line looks like this: |.

In sports, gymnasts perform on horizontal bars that stretch from side to side. In construction, builders check that floors and shelves are horizontal so things don't roll off. Scientists talk about horizontal movement when something travels across rather than rising or falling.

When you're reading this page, the lines of text run horizontally across the page from left to right. Understanding horizontal and vertical helps you describe how things are positioned in space, which matters in everything from building a treehouse to reading a map.