The Accordion (or Accordeon)
It is helpful to lead off with a description of the accordion, not because its use is so dominant in Celtic music (it’s not), but because many free-reed instruments are considered to be part of the accordion family. Accordions have a bellows with many folds; another consistent feature is a keyboard with from five to 50 keys. Accordions are unique because they may have a keyboard similar to that of a piano—with the familiar white and black keys—or they may have several rows of round button keys, as do the other instruments in the accordion family. To provide instant clues as to which type of keys are used, some people prefer a variant spelling for button key instruments (accordeons with an “e”) instead of accordions (with an “i”), the spelling always used when referring to accordions that have piano keys.
Depending on the model, the accordion can be described as single-action (diatonic type) or double-action (chromatic type). Double-action (unisonoric) accordions produce the same notes when the bellows are expanding as when they are contracting, while single-action (or bisonoric) accordions give different notes when the player expands and contracts the bellows. For reasons that are obvious if you’ve ever seen one in action, single-action models are sometimes called “push-pull” accordions.