Lots of buildings and everyday objects use symmetry because it looks appealing to the human eye. Watch the video below from BBC Bitesize KS2 Maths which shows the lines of symmetry on a square ...
I LOVE symmetry photography in my portrait, landscape and architectural work. Is it better than the rule of thirds?
Each diagonal splits a corner into two angles ... Using the rectangle's lines of symmetry, we can see the other angles which are equivalent and then fill in any missing angles in each triangle.