The rack unit size is based on a standard rack specification as defined in -310. The specifies a standard rack unit as the unit of height; it also defines a similar unit, (HP), used to measure the width of rack-mounted equipment. The standard was adopted worldwide as IEC 60297 Mechanical structures for electronic equipment – Dimensions of mechanical structures of the 482.6 mm (19 in) series, and defines the sizes for rack, subrack (a shelf-like chassis in which cards can be insert.