Why did Gaudí design his churches upside down, with hanging strings?

Gaudí hung his churches upside down because a hanging string finds the perfect arch by itself. A chain or rope that carries only its own weight settles into a curve called a catenary, and every part of it is pulled purely in tension. Flip that shape over and the pulls become pushes: an arch built along it is squeezed purely in compression, with no bending, exactly what stone and brick are good at. By hanging weighted strings from points marking his columns, Gaudí let gravity calculate shapes that would otherwise have taken endless equations.

The idea was old. In the 1670s Robert Hooke hid it in a Latin anagram, revealed after his death: 'As hangs a flexible cable so, inverted, stand the touching pieces of an arch.' Gaudí turned it into a design machine. For the church of the Colònia Güell near Barcelona, he hung hemp ropes loaded with little sacks of lead shot from a ceiling, at one-tenth scale, so the weights mimicked the real loads. Then he photographed the tangle, turned the photo upside down and painted the church over it.

The pay-off was a building that needed no flying buttresses. Gaudí despised them: Gothic architecture, he said, was 'a defective body held up on crutches'. His catenary arches carried their weight down their own curves.