It's always a pretty good idea to pay close attention to EVERYTHING you can see in your viewfinder, like the surroundings and backgrounds, and not just your main subject. Just of the (many!) aspects to watch out for when you are framing your photos is the horizon - which can actually mean ANY horizontal lines in your image, so don't think you've got away with it if you don't shoot landscapes :) Let's get going with the 5 "Rules" for the horizontal lines in your image!
This photograph captures the importance of line in art. The use of the fence with the patterned thin lines helps to accentuate the strong vertical lines of the bridge. The photograph makes your eyes move right to the intended focus and gives a sense of hopefulness.