That brings up an interesting point. This pattern can be denoted as p2. Here the mirror images are offset. This sort of symmetry is termed a glide . We can symbolize this pattern as pg.
Glides are often hard to see.
Summary of the one-dimensional space groups. Let the horizontal surface of the strip be a mirror plane. Run a two-fold axis down the length of the strip. This immediately gives us 21 patterns, three for each of the seven strip space groups. We can let the surface of the strip be a glide, and have two-fold axes in the plane of the strip but perpendicular to its length.