The most active of shapes use diagonal lines – the triangle is an eye-catching building block for your picture. Its three sides also introduce odd numbers into the photographic vocabulary. As well as triangular-shaped subjects, think about the structure of your photograph – are there three elements you could join together with imaginary lines to form a triangle?
Four sided shapes such as squares and rectangles mirror the four sides of the picture frame – there’s no conflict there, so the viewing experience isn’t as absorbing. However, they can be used alongside diagonals and triangles to produce a more exciting image.
~Digital Camera Magazine: Master Composition