In general, the dimensioning of racks, subracks, assemblies and PCBs is based on the inch, which corresponds to 25.4 mm. The height of the components mentioned is specified in height units(HE), the width in the pitch unit( TE), which is referred to in English as Horizontal Pitch (HP).
One Horizontal Pitch (HP) corresponds to 1/5", i.e. 5.08 mm. The pitch unit is used to divide the width of racks or subracks to determine the number of assemblies the rack can hold. Racks have a grid of holes spaced apart from the pitch unit into which the guide rails for the subassemblies can be mounted. Common pitch units have a multiple of 4 pitch units (HP), corresponding to 20.32 mm. The assemblies themselves also use Horizontal Pitch for their thickness specification.
In 19" systems, common cassettes use a nominal width between 10 HP and 21 HP.