A melodeon is a type of accordion. In most cases it refers to a button accordion with a diatonic scale.
Similar to a mouth organ or harmonica in the action of its reeds, the melodeon gives a different note when the bellows are expanded or contracted; thus it could be described as a single action or bi-sonoric instrument. In contrast, many other instruments of the accordion family are double-action or uni-sonoric as the same note is produced for each of the directions of the bellows.
A full octave can be played easily within the span of most hands. A one-row melodeon can be played in more than one key, but many notes will be missing from the scales thus derived. A number of buttons or keys on the bass side of the instrument activate valves for two or more reeds blown in parallel with each other in order to provide simple chords. These chords can be played in addition to the melody played on the treble side of the instrument. This chord/melody capability provided one basis of the patent for the original accordion.
