A diagonal line is a straight line that connects two non-adjacent corners of a shape, such as a rectangle or a square. It runs at an angle, rather than horizontally or vertically, creating a slant. Diagonal lines can be found in various geometric figures and are often used in art and design to create movement and depth. In mathematics, diagonal lines are important for understanding properties of shapes and calculating distances. For example, in a rectangle, the length of the diagonal can be found using the Pythagorean theorem. Diagonal lines also appear in graphs and charts, helping to illustrate trends and relationships between data points.