They seem simple: vertical curved structures that span an open space and may, or may not, support weight above it. Of course, arches are much more complicated than that, a complex balance of compression, stress, thrust, bracings, and transference. The Mesopotamians got the jump on them four thousand years ago, but it was the Romans who used them systematically in a wide range of structures, leading eventually to a worldwide adaptation of this most beautiful form. Read about the top five arches >