Perspective, straight lines and gradients.

[RobertArthur]

As a DIY enthusiast, you sometimes struggle with the spirit level: it’s always right (yes), but sometimes you have to cut corners to achieve a result that looks level to the human eye in a building where not all the walls and ceilings are perfectly level. That little problem faced by today’s DIY enthusiasts was one that the builders of temples and cathedrals in previous centuries had to deal with on a much larger scale, and for which they devised solutions.

The Piazza Inferiore di San Francesco (Lower Square of Saint Francis) in Assisi, which leads to the Basilica of St. Francis of Assis, is a rectangular, cloister-like square built in the 15th and 16th centuries. More tourists than pilgrims today. It is lined with porticoes featuring columns (perfectly vertical) and stone benches. This square slopes gently upwards, and when you sit down on one of those ancient little stone benches, it takes a moment to get used to the perspective.