from case to case, but the most commonly used locations (both at the rear of the case) and their advantages/disadvantages are: The top of the case, usually allowing the PSU's built-in fan to act as an auxiliary exhaust fan but causes the PSU to be fed air heated by the internal components of the case, thus causing PSU efficiency and lifespan degradation. The bottom of the case, with an (often filtered) vent in the bottom of the case allowing the PSU to draw cool air from the outside. Regardless of the mounting position, the PSU will usually be attached to the case with four screws to support its weight. Most cases include drive bays on the front of the case; a typical ATX case includes 5.25", 3.5" and 2.5" bays. In modern computers, the 5.25" bays are used for optical drives, the 3.5" bays are used for hard drives and card readers, and the 2.5" bays are used for solid-state drives. Buttons and LEDs are typically located on the front of the case; some cases include additional I/O ports, temperature, and processor speed monitors in the same area. Vents are often found on the front, back, top, left side panel, and sometimes on the right-side panel of the case. Regardless of their placement, their purpose is either to let cool air into the case or to let hot air out. Larger vents usually allow cooling fans to be mounted via surrounding threaded screw holes. Newer cases include mountings for larger 120mm or 140mm cooling fans for quieter operation than the 80mm fans formerly common. An I/O template, or I/O plate, fits around the motherboard's back panel of external ports. Sometimes provided by the case manufacturer, it is often replaced by the one that comes with the motherboard. Internal access Accessing the interior components of a modern ATX tower case is done by removing the side panels. Looking front-to-back, accessing the motherboard, PSU, drive bays, and most case fan installation points are done by removing the left side panel. Removing the right-side panel is done less often to access the space behind the motherboard mounting plate. This space is devoted to cable management, as cables routed in front of the motherboard may disrupt the flow of air within the case causing increased temperatures. BTX, an uncontemporary standard, has the main side panel on the right side as opposed to the left.