An impossible trident, also known as an impossible fork, a blivet, poiuyt, devil's tuning fork, etc., is a drawing of an impossible object (undecipherable figure), a kind of an optical illusion. It appears to have three cylindrical prongs at one end which then mysteriously transform into two rectangular prongs at the other end.
In 1964 D.H. Schuster reported that he noticed an ambiguous figure of a new kind in the advertising section of an aviation journal. He dubbed it a "three-stick clevis". He described the novelty as follows: "Unlike other ambiguous drawings, an actual shift in visual fixation is involved in its perception and resolution."  The word "poiuyt" appeared on the March 1965 cover of Mad magazine bearing the four-eyed Alfred E. Neuman balancing the impossible fork on his finger with caption "Introducing 'The Mad Poiuyt' " (the last six letters on the top row of many Latin-script typewriter keyboards, right to left). An anonymously-contributed version described as a "hole location gauge" was printed in the June 1964 issue of Analog Science Fiction and Fact, with the comment that "this outrageous piece of draftsmanship evidently escaped from the Finagle & Diddle Engineering Works" (although something else called a "hole location gauge" had already been patented in 1961).
The term "blivet" for the impossible fork was popularized by Worm Runner's Digest magazine. In 1967 Harold Baldwin published there an article, "Building better blivets", in which he described the rules for the construction of drawings based on the impossible fork. In December 1968 American optical designer and artist Roger Hayward wrote a humorous submission "Blivets: Research and Development" for The Worm Runner's Digest in which he presented various drawings based on the blivet. He "explained" the term as follows: "The blivet was first discovered in 1892 in Pfulingen, Germany, by a cross-eyed dwarf named Erasmus Wolfgang Blivet." He also published there a sequel, Blivets — the Makings.
- Andrew M. Colman, A Dictionary of Psychology, Oxford University Press, 2009, ISBN 0199534063, p. 369
- Article "Impossible Fork" at MathWorld
- The Hacker's Dictionary, article "Blivet"; It lists the impossible fork among numerous meanings of the term
- Brooks Masterton, John M. Kennedy, "Building the Devil's Tuning Fork", Perception, 1975, vol. 4, pp. 107-109
- Schuster, D. H., "A New Ambiguous Figure: A Three-Stick Clevis." Amer. J. Psychol. vol. 77, 1964, p.673, .
- "Doug Gilford's Mad Cover Site - Mad #93". Madcoversite.com. Retrieved 2010-10-22.
- "Hole location gauge - Patent 2998656". Freepatentsonline.com. 1961-09-05. Retrieved 2010-10-22.
- William Perl,"Blivet or Not", The Journal of Biological Psychology, 1969
- Gardner, Martin (1981). Mathematical Circus. Pelican Books. p. 5.
- Science, Sex, and Sacred Cows: Spoofs on Science from the Worm Runner's Digest, 1971, pp. 91-93