The impossible trident is one of the most notorious impossible figures. It was first seen by many when it appeared on the cover of the March 1965 issue of Mad magazine. The two halves of the figure seem perfectly reasonable in themselves. When the top part is covered, the bottom part is taken to be three separate cylinders or tubes. With the bottom part hidden, the foreground figure is interpreted as being built of flat faces making two rectangular prongs. The trouble is that these two aspects of the figure are totally incompatible. Somewhere in the middle, the foreground and background swap places and give rise to an irreconcilable paradox.
Over the years, countless adaptations of the trident have appeared with names such as the devil's fork, the three stick clevis, the blivit, the impossible columnade, the trichotometric indicator support, and, most extravagantly, the triple encabulator tuned manifold. Swedish artist Oscar Reutersvärd's mastery of such figures has led him to draw thousands of variations on the theme. When the figure is drawn long, it is easy to perceive locally as a three-dimensional object and to overlook its inherent inconsistency, because the contradictory clues are too well separated. When the figure is of medium length, the figure is easily interpreted as a three-dimensional object, and its impossibility is quickly perceived. If the prongs are very short, the two different interpretations vie for acceptance within the same local area; thus there is no consistent interpretation and the illusion breaks down. Some early writers commented that the impossible trident couldn't be built in any form in three dimensions. However, this has been shown to be false. In 1985, the Japanese artist Shigeo Fukuda made a 3-d model of the trident in the form of classical columns in which the illusion works – from one critical angle.
The origins of the figure are uncertain. It turns out that Mad magazine bought the illustration rights from a contributor who claimed that it was original; however, the magazine's management soon found out to their embarrassment that the figure had been previously published. It began to surface in several popular engineering, aviation, and science-fiction periodicals in May and June of 1964. D. H. Schuster published an article that same year in the American Journal of Psychology, which first brought the figure to the attention of the psychological community.