A reed rank inside accordions refers to a single full set of the reeds that are the means to achieve the instrument's sound range. These reed ranks are located in the reed chamber. Most accordions to this date typically have between 2 and 4 reed ranks on the treble side and between 3 and 5 reed ranks on the bass side. These can usually be selected individually or combined in various ways to provide a range of different timbres, by use of register switchesarranged by register from high to low. More of the top-line expensive accordions may contain 5 or 6 reed blocks on the treble side for different tunings, typically found in accordions that stress musette sounds. How many reeds an accordion has is specified by the number of treble ranks and bass ranks. For example, a 4/5 accordion has 4 reeds on the treble side and 5 on the bass side. A 3/4 accordion has 3 reeds on the treble sides and 4 on the bass side. Reed ranks are classified by either organ 'foot-length' stops or instrument names. Visually, they each have a fixed dot in a three-level icon as displayed in the photo on the right and tables below. These icons display when more than one reed-rank is in use.
The pitch of a single bank of reeds is traditionally defined in a similar manner to the organ stops of a pipe organ. A bank that sounds at unison pitch when keys are depressed is called 8′ pitch: alluding to the length of the lowest-sounding organ pipe in that rank, which is approximately eight feet. For the same reason, a stop that sounds an octave higher is at 4′ pitch, and one that sounds an octave lower than unison pitch is at 16′ pitch. Most reed registers are normally in relative octave tuning, but rarely, some instruments have a reed bank tuned to a perfect fifth relative to the 8′ stop. This is a similar arrangement to stops for a pipe organ. On accordions with two 8′ ranks, one is tuned a fraction of a semitone higher than the other ranks. This causes beats when the two 8′ ranks sound together, creating a distinctive tremolotimbre. Tunings where the difference between the two is small and the beats are less noticeable are referred to as "dry", whereas those where the difference is large are referred to as "wet". Accordions with three 8′ ranks have the third tuned the same distance below the center, doubling the effect of the beats when all three play.
Register switches
Register switches select combinations of reed ranks to produce contrasting timbres. Most accordions have automatic or preset switches, similar to voice selection on an electronic keyboard, or to a preset combination action in a pipe organ. These switches control which reed ranks are enabled or disabled : some switches enable a single reed rank, others enable several simultaneous reed ranks. In general, the formula for the number of potential switch combinations is one less than 2 to the number of unique reed blocks that are within the accordion. For example, if an accordion has 3 reed blocks, there are potentially 2-1 = 7 combinations, though "less useful" ones are often omitted. Unlike individual organ stops, only one combination is active at any given time. Here are a few examples of right-hand manual switches on a typical large accordion. es controlling individual reed ranks|alt=Set of two-wayrocker switches controlling individual reed ranks for the treble keyboard of an Excelsior accordion. Instead of automatic switches, some accordions have individual switches for the reed ranks that can be used in any combination, like organ stops. In addition to the master switch located with the other switches shown above, professional-grade accordions often have at least one extra master switch: either a chin master at the top of the instrument or palm master switch located at the side of the keyboard. These allow for faster changes to the register while the right hand is playing the melody.
Cassotto
High-end accordions often have a feature called a cassotto, also referred to as a "tone chamber", in the treble reed section. In this design, certain reed sets are mounted at a 90° 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 its harmonics and creating a distinctively mellow, refined sound. The sound of cassotto bassoon 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 90° angle. To do this properly, each rod and pad must be positioned precisely in relation to its perpendicular counterpart. Because of the considerable extra time required for the cassotto's construction and adjustment, cassotto accordions cost considerably more than similar non-cassotto models.
