Lines can be effective elements of composition, because they give structure to your photographs. Lines can unify composition by directing the viewer’s eyes and attention to the main point of the picture or lead the eyes from one part of the picture to another. They can lead the eyes to infinity, divide the picture, and create patterns. Through linear perspective, lines can lend a sense of depth to a photograph. (Linear perspective causes receding parallel lines to appear to converge in the picture. This allows you to create an illusion of depth in your pictures.)
The viewer’s eyes tend to follow lines into the picture (or out of the picture) regardless of whether they are simple linear elements such as fences, roads, and a row of phone poles, or more complex line elements, such as curves, shapes, tones, and colors. Lines that lead the eye or direct attention are referred to as leading lines. A good leading line is one that starts near the bottom corner of the scene and continues unbroken until it reaches the point of interest. It should end at this point; otherwise, attention is carried beyond the primary subject of the photograph. The apparent direction of lines can often be changed by simply changing viewpoint or camera angle.
Vertical, diagonal, horizontal, and curved lines create different moods. Vertical lines communicate a sense of strength, rigidity, power, and solidarity to the viewer. On the other hand, horizontal lines represent peace, tranquillity, and quietness. A generally accepted practice is to use a vertical format for pictures having predominantly vertical lines and horizontal format for pictures having predominantly horizontal lines. Again, this is a generally accepted practice, NOT a rule.