The document discusses different types of structures and their characteristics. It describes frame structures as the most common type, composed of long elements joined together. Shell structures are made of a thin outer layer and can be empty or filled. Arches were invented by Romans, with the semicircular arch being most used, supported by a keystone stone at the top. Mass structures are simply made of clumped material and are normally thick, with mountains and coral reefs as natural examples. Suspension structures hold elements with cables from the top of tall columns, with the space between columns called the span.