A shape has reflection symmetry (also called mirror symmetry or line symmetry) if you can draw a line through it so that one side is the mirror image of the other. The line is the line of symmetry, or axis. The paper test: fold the shape along the line. If the two halves land exactly on top of each other, the line is an axis of symmetry.

Everyday shapes are full of axes. Take capital letters in a plain sans-serif font:

• A, M, T, U, V, W, Y have one vertical axis.
• B, C, D, E, K have one horizontal axis.
• H, I, X and an oval O have two axes (an O drawn as a perfect circle has infinitely many).
• F, G, J, L, P, R have none. (Fonts differ, so check your own.)

Faces, butterflies and most animals are only roughly symmetrical: a real face is never an exact mirror image, which is why a photo of someone next to its own mirror image looks like two slightly different people.