Bilateral symmetry (also called axial or mirror symmetry) means one half of the composition is the mirror image of the other about a central axis.

Examples in architecture: Taj Mahal (bilateral symmetry on all four sides), most classical Greek temples, and many government buildings.

Other design principles: Rhythm = repetition of elements creating movement. Radial balance = elements arranged around a central point. Contrast = juxtaposition of opposing elements (light/dark, old/new).