# Can you eyeball a perfect square?

The width is fixed at 200 mm. Raise the top edge until the rectangle becomes a perfect square. On each round, the master caliper gauge drops over the slab and measures your error.

[Open the instrument](https://mariuscomper.uk/patratul-perfect/en/)

## How it works

1. Adjust the shape's height by dragging the top edge or using the step buttons.
2. When the shape looks perfectly square, press “Check”.
3. The master caliper gauge drops over the slab, revealing the H/W ratio, millimetric delta, and percentage error.
4. Across five rounds (clean outline, solid block, horizontal stripes, 45° diamond, and flanking pillars), your attempts accumulate into a workshop blueprint diagram.

## Why the vertical looks longer

In 1858, Wilhelm Wundt described the horizontal-vertical illusion: vertical extents appear 5% to 15% longer than identical horizontal extents. Human binocular vision operates within a panoramic horizontal field; vertical eye movements demand greater effort and occupy a larger fraction of the vertical visual meridian. When constructing a square by eye, we consistently stop raising the top edge too soon.

Concept and code: Marius Comper, 3 September 2026.