One of the earliest structural designs is the post and lintel system. It is comprised of at least two vertical members (the posts) and at least one horizontal member (the lintel). These are evident in Stonehenge. Here is an example from the Acropolis that features columns (posts). The post supports the weight of what is above it - compressive strength. The lintel relies on its ability to withstand bending - tensile strength. In stone the columns cannot be too far apart or the lintel will break.