Symmetry 

Symmetry is a good way to move away from the standard ‘rule of thirds’ shot and bring something a little different to an image but it does need to be done well to ensure it works effectively. Things that are almost symmetrical will often fail, yet occasionally you can produce a nice image with a topic that is seemingly symmetrical. As with all things related to photography trial and error is the best method of all to gain a more solid understanding.

This shot of Waterton Lakes National Park in southern Alberta is not perfectly symmetrical but gets away with the minor differences because of the overall aesthetic.  

To work well symmetry needs to be virtually perfect, you could argue that if it's not then it's not symmetry I guess.  I'll be the first to admit it's not something I've got even close to mastering, and that is the joy of photography, you can always find something new to try, a new way to shoot, a new method or topic. This gallery represents some of my efforts at symmetry thus far, it appears I am still in need of practice.   ​

Diagonal symmetry works equally well though there are few occasions where it occurs naturally without you specifically seeking it out unless shooting abstracts and 'moving camera or lens' shots, when it's much easier to facilitate diagonal symmetry.  I include this only as a reminder that symmetry is not restricted to horizontal and vertical planes.