(such as raised floor tile dimensions), these dimensions have become quite common. The extra width and depth enables cabling to be routed with ease (also helping to maintain bend-radius for fibre and copper cables) and deeper equipment to be utilised. A common feature in IT racks are mounting positions for "Zero-U" accessories, such as PDUs (power distribution units) and vertical cable managers/ducts, that utilise the space between the rear rails and the side of the rack enclosure. The strength required of the mounting posts means they are invariably not merely flat strips but actually a wider folded strip arranged around the corner of the rack. The posts are usually made of steel of around 2 mm thickness (the official standard recommends a minimum of 1.9 mm), or of slightly thicker aluminum. Racks, especially two-post racks, are often secured to the floor or adjacent building structure so as not to fall over. This is usually required by local building codes in seismic zones. According to Telcordia Technologies Generic Requirements document GR-63-CORE, during an earthquake, telecommunications equipment is subjected to motions that can over-stress equipment framework, circuit boards, and connectors. The amount of motion and resulting stress depends on the structural characteristics of the building and framework in which the equipment is contained, and the severity of the earthquake. Seismic racks rated according to GR-63, NEBS Requirements: Physical Protection, are available, with Zone 4 representing the most demanding environment. GR-3108, Generic Requirements for Network Equipment in the Outside Plant (OSP), specifies the usable opening of seismic-compliant 19-inch racks. Rails (slides) Heavy equipment or equipment which is commonly accessed for servicing, for which attaching or detaching at all four corners simultaneously would pose a problem, is often not mounted directly onto the rack but instead is mounted via rails (or slides). A pair of rails is mounted directly onto the rack, and the equipment then slides into the rack along the rails, which support it. When in place, the equipment may also then be bolted to the rack. The rails may also be able to fully support the equipment in a position where it has been slid clear of the rack; this is useful for inspection or maintenance of equipment which will then be slid back into the rack. Some rack slides even include a tilt mechanism allowing easy access to the top or bottom of rack-mounted equipment when it is fully extended