More expensive accordions have a feature called a Cassotto (Italian for "box"), also referred to as a "tone chamber", in the treble (right-hand) reed section. In this design, certain reed sets (usually one set of middle reeds, and the set of low reeds) are mounted at a 90-degree angle to the remaining reeds. The sound from these specially-mounted reeds must then travel farther, and along a different path, before leaving the instrument, muting the harmonics (partials) of those reeds and creating a distinctively mellow, refined sound. The sound of cassotto bassoon (low) reeds is particularly favored by jazz accordionists.
The cassotto design requires a sophisticated treble mechanism where each key must open and close air passages not only for reeds mounted at the traditional angle, but also for air passages at a relative 45-degree angle (for the cassotto reeds). To do this properly, each rod and pad must be positioned precisely in relation to its right-angle counterpart.
Read more about this topic: Accordion Reed Ranks And Switches