Rule of Three
Rule of Three is a representation of the Penrose Triangle (which is sometimes referred to as a Tribar). Although it was first created by the Swedish artist Oscar Reutersvärd in 1934, Sir Roger Penrose and his father, Lionel, independently devised this and other impossible figures in the 1950s. Their discoveries are known to many as the underlying constructions of several works by M C Escher.
At first glance it seems to be a perfectly normal triangle made from three bars with square cross-section. As you follow the faces around it becomes obvious that such pieces can never fit together.
It is an unusual shape for an afghan but quite useful if you make it long enough to cover your feet. Depending on the size, you could also use it like a shawl.
The instructions allow you to make this in any yarn, to any size you want. It is very easy to make as only one yarn is used at a time.