500 Series Racks: All of those modules have to live somewhere, and that’s where the chassis (frame) or Lunchbox (an API trademark) comes in. The chassis has slots for two or more 500 Series modules. It provides not only a slot for the module to live in but also power for the module, as well as inputs and outputs for getting signal in and out of the module.  Some chassis are tabletop units, while others mount into racks. They range in size from 2-slot (which can hold two single-width modules or one double-width module) to 10-slot, which can hold up to 10 single-width modules or a mix of single- and double-width modules.