Unisonoric or bisonoric?
The term accordion is not entirely unambiguous. Today it can refer broadly to free-reed instruments whose treble keyboard is mounted at an angle, but it is also used for particular forms. The word harmonica is sometimes used as a synonym and sometimes to distinguish other instrument families.
There are also many regional names for these instruments. An important technical distinction, however, is whether an instrument is unisonoric or bisonoric.
If each button produces the same note when the bellows are opened and closed, the instrument is unisonoric. These instruments are generally easier to learn because the note arrangement does not change with the direction of the bellows.
If most buttons produce different notes depending on whether the bellows are opened or closed, the instrument is bisonoric. This system reduces the amount of mechanism required: ten buttons can produce twenty notes.
A piano accordion is always unisonoric. Select the system used by your instrument.
