Can you eyeball a perfect square?
The width is fixed at 200 mm. Raise the top edge until the rectangle looks like a perfect square. On each round, the master caliper gauge drops over the slab and measures your error.
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Clean outline
Round 1 of 5Clean contours isolate the baseline illusion: vision inflates vertical extent by 5% to 10%.
Workshop blueprint
Why the vertical looks longer
In 1858, psychophysicist Wilhelm Wundt documented a persistent visual asymmetry: a vertical line appears 5% to 15% longer than an identical horizontal line. Human vision operates within a wide, panoramic horizontal field; scanning upward demands greater vertical saccadic effort and spans a higher fraction of the eye's vertical meridian.
The practical result is immediate: when you construct a square by eye, you stop raising the top edge too soon. The slab looks square while it is still several millimetres shorter than its base.
In 1867, Hermann von Helmholtz proved that transverse horizontal hatching amplifies this perceived height. In 1886, Ernst Mach showed that rotating the shape 45 degrees shifts visual attention to the diagonals, compressing perceived aspect ratio once again.
Measurement limits: This instrument tests visual proportion estimation on your screen using touch, mouse, or keyboard input. Display latency, viewing angle, and screen orientation may introduce minor variations. It does not replace an optometric examination.