The most important four lines of any image are the ones that are often least recognized consciously – the frame. (Non-rectangular frame shapes are quite rare, while they exert a modified influence over the visual dynamics at work within the image, the influence of the frame is still extreme.) Part of learning to make successful compositions is learning to become more conscious of the frame and how to use the forces it exerts on your images for desired effects.
Use of the frame qualifies the concerns of the author. The frame delimits what’s in or out of an image. The frame exerts a strong visual influence on the viewer. Its proportion exerts a psychological influence on the viewer and may guide him or her to looking in specific ways. It determines orientation. It can be used to emphasize and deemphasize elements contained within it. It can contain, modify, or create shape. It determines relative position. Dividing it can produce many strong visual effects.
The frame is incredibly important. Its power should never be overlooked or underutilized.