Three prongs or two?
The blivet, the poiuyt, the impossible trident: a fork that changes its mind halfway down.
How this site works. Every fact is in our own words and linked to its source, beside it. Where good sources disagree, we show each version in a “Sources differ” box. Every drawing is our own.
Many names
The devil’s fork is also called the blivet; in his 1964 paper D. H. Schuster called it a “three-stick clevis”. Michael Bach
Other names include the impossible fork, the devil’s pitchfork and the poiuyt. MathWorld The Illusions Index also calls it the impossible trident or the devil’s tuning fork. Illusions Index
What goes wrong
At one end it seems to have three prongs; at the other, only two. Illusions Index Each prong looks normal; it is working out how they join that goes wrong. MathWorld
Where it came from
Schuster, an American psychologist, based it on an advertisement he saw in an aviation journal. Illusions Index It was first published in The American Journal of Psychology in 1964. Illusions Index
Mad magazine made it a running joke, calling it the “poiuyt”: the third row of a keyboard typed from right to left. MathWorld In Mad, throughout the 1960s, the poiuyt stood for anything absurd or impossible. MathWorld
The artist Roger Hayward worked the fork into a picture of Greek columns. MathWorld
Draw one in three steps
- Michael Bach’s way to draw one: start with six evenly spaced upright lines and give them “hats”, and you get three towers or a three-pronged fork. Michael Bach
- Cover the top, draw connecting curves at the bottom, and you get the bottom of a square bar bent into a U. Michael Bach
- Uncover the top and you see the two sensible readings don’t fit together. Michael Bach
Bach’s impossible “contraption” also includes three “hexnuts” and a “Penrose frame”. Michael Bach
Next: the impossible cube →