Guitars have a circular hole so that the sound made by the strings can resonate inside the hollow body of the guitar and be amplified as it bounces back. Violins, on the other hand, have F-holes, those squiggly things on either side of the string.
They're called F-holes because they look like the cursive small f. However, the holes on either side of the violin weren't always shaped like that. Its earliest ancestor was the fithele (where the word "fiddle" came from) in the 10th century which had circular holes on either side. The design eventually evolved through the centuries, until Renaissance craftsmen found that the cursive f-shape produced the best sound for the violin.
The video above proposes that the reason for the F-holes was because of the obsession of Renaissance masters over perfection such as the fibonacci sequence or the golden ratio. And possibly, while somebody had been peeling a clementine, they found that it produced the shape of a cursive f.
However, other researchers, such as Nicholas Makris and his colleagues at MIT, say that 16th century violin makers most likely came upon the shape through trial and error, whether or not they understood the mathematical principles behind the design.
The researchers have found that elongated sound holes allow more sound to be produced by the violin and that explains why the design has remained the same for 500 years.
(Video credit: The Celtic Goldsmith Bellchamber)