For example, a case designed for an ATX motherboard and power supply may take on several external forms such as a vertical "tower" (designed to sit on the floor, height > width), a flat "desktop" (height < width) or "pizza box" (height ≤ designed to sit on the desk under the computer's monitor). Full-size tower cases are typically larger in volume than desktop cases, with more room for drive bays, expansion slots, and custom or all-in-one (AIO) water cooling solutions. Desktop cases—and "mini-tower" cases under about high—are popular in business environments where space is at a premium.