A computer case, also known as a computer chassis, is the enclosure that contains most of the hardware of a personal computer. The components housed inside the case (such as the CPU, motherboard, memory, mass storage devices, power supply unit and various expansion cards) are referred as the internal hardware, while hardware outside the case (typically cable-linked or plug-and-play devices such as the display, speakers, keyboard, mouse and USB flash drives) are known as peripherals. Conventional computer cases are fully enclosed, with small holes (mostly in the back panel) that allow ventilation and cutout openings that provide access to plugs/sockets (back) and removable media drive bays (front). The structural frame (chassis) of a case is usually constructed from rigid metals such as steel (often SECC — steel, electrogalvanized, cold-rolled, coil) and aluminium alloy, with hardpoints and through holes for mounting internal hardware, case fans/coolers and for organizing cable management. The external case panels, at least one of which are removable, cover the chassis from the front, sides and top to shield the internal components from physical intrusion and dust collection, and are typically made from painted metallic and/or plastic material, while other materials such as mesh, tempered glass, acrylic, wood and even Lego bricks have appeared in many modern commercial or home-built cases. In recent years, open frame or open air cases that are only partly enclosed (with freer ventilation and thus theoretically better cooling) have become available in the premium gaming PC market. Sizes and terminology Cases can come in many different sizes and shapes, which are usually determined by the form factor of the motherboard since it is physically the largest hardware component in most computers. Consequently, personal computer form factors typically specify only the internal dimensions and layout of the case. Form factors for rack-mounted and blade servers may include precise external dimensions as well since these cases must themselves fit in specific enclosures. For example, a case designed for an ATX motherboard and power supply unit (PSU) 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.