Ever since sticky pistons were released in Beta 1.7, So many different doors could be made. One of the very popular ones is the original, 2x2 Sliding Piston Door. Any object could be used to make the door, such as glass, stone, Iron Blocks, etc. As of 1.8, Glass Panes were released. This was another block that could be used in the door. Here's a few pictures of this cool feature, creating a "twisting" motion.
In This Image,^^ The Doors are open, using 2 panes per side, facing the wall.
In This Image,^^ The Doors are closed, using 4 panes total, looking like 2 complete panes, facing the outside/inside.
As the Panes are pushed, they connect and "rotate" as they connect with other blocks, creating a neat twisting effect.