Because it flips rather than glides, it is called an indirect or improper operation. Rotation and translation which involve displacement are called direct or proper operations. The image reverses, but in addition it glides. Instead of a mirror line, we have a glide line , or, a line that marks the path of a translation. Glide lines can run vertically, horizontally, or diagonally.
Mirror reflection is a special case of glide reflection .
Mirror reflection is glide reflection with zero glide, as translation is a special case of rotation. A symmetrical figure has repetative parts. A mirror through the center leaves their appearance unaltered. A figure is symmetrical if a symmetry operation leaves the appearance unaltered. N and Z are symmetrical by rotation.