single rack, it is impractical for each one to have its own separate keyboard, mouse, and monitor. Instead, a KVM switch or LOM software is used to share a single keyboard/video/mouse set amongst many different computers. Since the mounting hole arrangement is vertically symmetric, it is possible to mount rack-mountable equipment upside-down. However, not all equipment is suitable for this type of mounting. For instance, most optical disc players will not work upside-down because the driving motor mechanism does not grip the disc. Rack types 19-inch server racks can vary in qualities. A standard 19-inch server rack cabinet is typically 42u in height, wide, and deep. Newer server rack cabinets come with adjustable mounting rails allowing the user to place the rails at a shorter depth if needed. There are a multitude of specialty server racks including soundproof server racks, air conditioned server racks, NEMA rated, seismic rated, open frame, narrow, and even miniature 19-inch racks for smaller applications. Cabinets are generally sized to be no wider than the standard floor tiles used in most data centers. Racks carrying telecom equipment like routers and switches often have extra width to accommodate the many cables on the sides. Four-post cabinet racks Four-post racks allow for mounting rails to support the equipment at the front and rear. These racks may be open in construction without sides or doors, or may be enclosed by front and/or rear doors, side panels, and tops. Most data centers use four-post racks. Two-post relay racks Two-post racks provide two vertical posts. These posts are typically heavy gauge metal or extruded aluminum. A top bar and wide foot connect the posts and allow the rack to be securely attached to the floor and/or roof for seismic safety. Equipment can be mounted either close to its center of gravity (to minimize load on its front panel), or via the equipment's front panel holes. The Relay Racks name comes from early two-post racks which housed telephone relay and switching equipment. Two-post racks are most often used for telecommunication installations. ATA road case racks 19-inch equipment that needs to be moved often or protected from harsh treatment can be housed in an Air Transport Association of America (ATA) approved road case sometimes also referred to as flight case. Road cases typically have plywood sides laminated with polyvinyl chloride (PVC), extruded aluminum edges, steel corners, handles, and latches. Larger cases typically have