A desktop computer is a personal computer (PC) in a form intended for regular use at a single location, as opposed to a mobile laptop or portable computer. Early desktop computers are designed to lay flat on the desk, while modern towers stand upright. Most modern desktop computers have separate screens and keyboards. Prior to the widespread use of microprocessors, a computer that could fit on a desk was considered remarkably small. Early personal computers, like the IBM PC, were 'desktop' machines, with a horizontally oriented computer case, usually intended to have the display screen placed on top to save space on the desktop. In modern usage the word 'desktop' usually refers to tower cases that are in fact more often located on the floor under the desk than on a desk. Technically speaking desktop and tower computers are two different styles of computer case that use desk space in varying ways.