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 in triangles or parallelograms. Diagonal lines create a sense of movement and direction, often making a composition more dynamic. In geometry, diagonal lines can also refer to the lines that connect opposite corners of a rectangle or square. These lines help in dividing shapes into smaller sections and are essential in understanding concepts like symmetry and proportions. Diagonal lines are commonly used in art and architecture to enhance visual interest.