Barbier's theorem

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What Wikipedia says

In geometry, Barbier's theorem states that every curve of constant width has perimeter π times its width, regardless of its precise shape. This theorem was first published by Joseph-Émile Barbier in 1860.

Excerpt from the Wikipedia article “Barbier's theorem”, under the CC BY-SA 4.0 license. Read the article on Wikipedia

Image: HB, CC BY-SA 4.0, via Wikimedia Commons.