Diagonal lines are straight lines that slant at an angle, connecting two points that are not aligned vertically or horizontally. They can be found in various shapes and designs, such as triangles and parallelograms. Diagonal lines create a sense of movement and direction, often making objects appear dynamic and engaging. In geometry, diagonal lines are important for understanding the properties of shapes. For example, in a rectangle, the diagonals connect opposite corners and can be used to calculate the area. Diagonal lines also play a significant role in art and architecture, guiding the viewer's eye and adding depth to compositions.