Melodeon

A Melodeon can be defined as a one-row diatonic button accordion used primarily in Cajun & Irish music. Although its melody notes are all in one key, by playing with the tonic on the draw, an additional key (technically the mixolydian mode) can be played. This is much like the way blues harmonica players play their instruments in "cross-harp." However, the definition for Melodeon is different from place to place; for example in England it refers to any form of button accordion, while the definition above is the Irish one.