Diagonal lines are straight lines that slant at an angle, rather than running horizontally or vertically. 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 art and design, diagonal lines are used to draw the viewer's eye and create depth. They can also be seen in everyday objects, like the edges of a roof or the slope of a hill. Understanding diagonal lines helps in recognizing patterns and structures in both nature and human-made environments.