Periaktoi are an ancient means of representing location and changing scenery. They are three sided rotating towers that have different scenic representations on each side. Traditionally, they are used in a fixed line, so rotating them creates a new backdrop for your set. That isn’t the only way to use them, however. They can be free moving, staggered, or any configuration that works for your show, but the concept is the same, different images on each side make for easy scene changes.