A device for regulating air pressure in wind instruments such as the concertina, melodeon and accordian. It is a number of folds of typically leather over card or wood frames. Cheaper models may use paper or vinyl instead of leather. The number of folds has an impact on playability, as does the suppleness of the material. Obviously it needs to be airtight.
Air is taken in on the out stroke (the pull) and the bellows expand. This may be done by simply holding an air key (which is silent) or by holding down a note. Air is then repeatedly discharged and recharged as notes are played and the bellows are pushed closed and then opened again.
Finally, the air key is used to release the final unused air in the bellows.
On instruments without air keys, players sometimes remove the least used reed in the push direction as a substitute.