An apartment (called a flat in the UK, Australia, and India, or a unit in Australia) is a self-contained housing unit within a larger building, with its terminology varying by region—such as apartment in North America and flat in the UK. The housing tenure of these units spans widely, from renting from a private landlord to owning a share via a condominium (strata title) or housing cooperative, and in England and Wales, some flat owners hold a "share of freehold" in the company that owns the building itself. The structures that house them go by names like apartment complex, tower block, or high-rise, with a skyscraper typically defined as a building with fifty or more stories. Many of these buildings are mixed-use, combining retail or offices on the lower floors with residential apartments above. The rise of high-rise apartments became possible thanks to the invention of the elevator and cheaper building materials, which transformed urban living and made dense, vertical housing a global norm.