Perpendicular means forming a right angle, or meeting at exactly 90 degrees. When two lines are perpendicular to each other, they create the shape of a perfect corner, like where a wall meets the floor or where the vertical and horizontal lines cross on a piece of graph paper.

Think about the letter T: the vertical line is perpendicular to the horizontal line. The legs of most tables stand perpendicular to the floor. Street signs are perpendicular to their poles. Basketball hoops are perpendicular to the backboard.

In geometry, perpendicular lines are especially important because they create squares and rectangles, which are fundamental shapes in mathematics and construction. Builders use levels and squares to ensure walls are perpendicular to floors and to each other. Without perpendicular angles, buildings would be crooked and unstable.

When you see two lines that look like a plus sign or the corner of a book, you're seeing perpendicular lines. Mathematicians sometimes write a special symbol (⊥) to show that two lines are perpendicular.