Three Round Prongs That Become Two Flat Bars. Count Them Both. Both Are Right.

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You are looking at the Devil’s tuning fork · an impossible figure first drawn in the 1960s by vision researcher D.H. Schuster and popularised in a classic 1965 American Journal of Psychology paper. The figure shows a tuning fork · or what looks like one. Count the prongs at the right end: you see three cylindrical prongs extending to the right, clearly rounded in cross-section. Now follow the prongs leftward. They seem to merge into two flat bars at the left end of the figure · two rectangles where there should be three cylinders. The transition happens gradually, but there is no single point where a third prong obviously disappears. Nothing is missing; nothing is hidden; and yet the figure has three prongs on one side and two on the other. The topology is impossible.

What the Illusion Looks Like

Draw three parallel cylindrical prongs at the right side of a figure. Each prong is drawn as two horizontal lines (the top and bottom of the cylinder’s silhouette) plus a small arc at the right end. Now extend those same lines to the left. Where the three cylinders should continue into three cylinders, they instead converge · the lines cross-connect so that the three-prong end becomes a two-bar end. The middle prong’s lines merge with neighbouring prongs’ lines in a way that is locally unremarkable but globally impossible.

You see: three prongs on the right, two bars on the left, and a smooth transition between them. Nothing looks wrong at any one place. Only when you count both ends do you realise the figure violates basic topology.

Why It Works: Edge Grouping Without Whole-Object Verification

The Devil’s tuning fork exploits a specific weakness in edge grouping · the visual system’s process of deciding which lines in a drawing belong to which 3D object.

Local edge grouping produces consistent interpretations at each end. At the right end of the figure, your visual system groups the lines into three cylindrical prongs · each prong has its own top and bottom silhouette. At the left end, it groups the lines into two rectangular bars · each bar has its own two edges.

The middle region is drawn ambiguously. The specific lines in the middle of the figure can be grouped in two ways: (a) as the continuation of three prongs, or (b) as the continuation of two bars. Depending on which end you are looking at, your visual system makes one grouping or the other at each moment.

Global object identity is never verified. Your visual system never checks that the three-prong interpretation at the right end is consistent with the two-bar interpretation at the left end. It accepts each local grouping as a local fact and moves on. The impossibility only emerges if you consciously count the prongs at each end and notice the mismatch.

The Tuning Fork Is Actually a Trident

Purists sometimes distinguish the Devil’s tuning fork from the impossible trident · in one variant, the figure has three prongs and two handles (more like a fork); in another, it has three prongs and three handles (more like a real trident, but with an impossible internal structure). Both versions exploit the same cortical principle.

A Harder Variant

Below is a Devil’s tuning fork figure at difficulty 3 · cleaner cross-section rendering, more prongs on one end. The impossibility is more striking.

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Why the Figure Is So Disorienting

Looking at the Devil’s tuning fork for more than a few seconds can be quite uncomfortable. Many viewers report headaches or mild nausea after prolonged study. This is unusual among optical illusions · most cause no such discomfort.

The Schuster-Shepard Legacy

D.H. Schuster and Roger Shepard published a series of papers on impossible figures in the 1960s and 1970s, helping to formalise the concept of “cognitive contradiction” in visual perception. Shepard himself went on to influence cognitive science broadly, introducing mental rotation, Shepard tones, and other classic stimuli.

Where the Devil’s Tuning Fork Appears

• Engineering textbooks as a cautionary tale. Some mechanical engineering and drafting textbooks include the Devil’s tuning fork as an example of why drawings must be checked globally, not just locally · a reminder that a figure that looks fine at each point can be structurally impossible.
• Mathematics education. The figure appears in topology and graph theory courses as a visual example of an object that has inconsistent boundary topology.
• Psychology demonstrations. As a stimulus for studying edge grouping, 3D reconstruction, and cortical limits. One of the standard figures in perception-lab demonstrations.
• Advertising and art. The figure (or its blivet variant) appears occasionally in advertising · often for products that want to suggest “more than you expect” or “things aren’t what they seem.”
• Internet memes. The figure is popular on image boards and in online puzzle communities, usually presented as a challenge to “count the prongs” or “find what’s wrong with this figure.”

Test Yourself on 50 More Illusions

The Devil’s tuning fork is one of more than 50 classical illusions on PlayMemorize. Each round draws a deterministic SVG scene and asks one grounded question: which is larger, which is brighter, which is actually parallel. The reveal overlay shows the true geometry plus a one-line “why it works” caption.

• Keep playing Devil’s Tuning Fork → · the standalone game, pinned to this one figure with fresh seeds each round
• Play Illusions → · spot the tricks across size, colour, orientation, and impossible figures
• Play Spatial → · train mental rotation and area estimation
• Play Matrix → · abstract pattern reasoning under time pressure