We begin by introducing names for certain features that may occur in symmetrical patterns. To each such feature of the pattern, there is a corresponding feature of the quotient orbifold, which we will discuss later. A mirror is a line about which the pattern has mirror symmetry.
Mirrors are perhaps the easiest features to pick out by eye.
At a crossing point , where two or more mirrors cross, the pattern will necessarily also have rotational symmetry. An n -way crossing point is one where precisely n mirrors meet. We obtain a mirror string by starting somewhere on a mirror and walking along the mirror to the next crossing point, turning as far right as we can so as to walk along another mirror, walking to the next crossing point on it, and so on. Suppose that you walk along a mirror string until you first reach a point exactly like the one you started from. For example, look at a standard brick wall.