In standard fonts, the letters A, M, T, U, V, W and Y each has a vertical line of symmetry that divides it into two corresponding mirror images. B, C, D, E and K have horizontal lines of symmetry. Both horizontal and vertical lines of symmetry occur in H, I and X. Meanwhile, F, G, J, L, N, P, Q, R, S and Z have no lines of symmetry.
The letter O is unique because it has an infinite number of lines of symmetry. If it is folded over on any diagonal, the two halves of the letter are congruent mirror images.
A few lower case letters that resemble their upper case partners also have lines of symmetry. These smaller versions are c, o, v, w and x. Symmetry of letters depends upon the font that is used. For instance, depending upon how it is written, a lower case "u" can have a vertical line of symmetry.
Certain words have lines of symmetry, as well. When written in block capital letters, ICEBOX, DECIDED, EXCEEDED, CHECKBOOK, HIDE and CHOICE all have horizontal symmetry.
A few capital letters have "rotational symmetry," which means they are identical if they are turned 180 degrees. These letters are H, I, O, S, X and Z.