A melodeon or diatonic button accordion is a member of the free-reed aerophone family of musical instruments.

To simplify matters and avoid ambiguity, in the remainder of this article the term diatonic button accordion, or DBA, will be used.

When the bellows are pressed, every button produces a note from the major triad of the home key; in this case, the pattern CEG repeats itself throughout the keyboard.

For detailed diagrams of typical note layouts on various types of diatonic button accordion (DBA), see melodeon.net.

A one-row DBA has the advantages of being light and compact, but is by its nature limited to the notes of a single diatonic scale.

Since the mid-to-late 19th century, instruments have been produced with more than one row in order to give players a greater choice of scales and tonalities.

Styles of play have developed in which row-crossing allows the bass side to be used to maximum effect, and the number of changes of bellows direction greatly reduced.

Another feature designed to increase the flexibility of fourth-apart systems is the inclusion of notes that lie beyond the diatonic scales of each row, or "accidentals".

Using the accidentals, and with the added modification of a Gleichton (unisonoric second-octave tonic in the centre of the middle row), this system allows players to obtain a fully chromatic scale – albeit in one direction only (draw).

[9] Another use of such additional short rows, or half-rows, is to provide reversals (see above) to give the player greater flexibility.

Some modern players, particularly in France, are driving a trend towards instruments with more complex bass systems, with as many as 16 or even 18 buttons.

Sometimes these more elaborate systems will diverge from the single-action principle, and may feature bass notes only instead of bass-chord pairs of buttons.

These include the Russian garmon, the Steirische Harmonika[notes 1] or Slovenian-style accordion that is popular in Alpine regions of Europe, the Swiss Schwyzerörgeli and the Basque trikitixa; the last two combine single- and double-action (bisonoric and unisonoric) features.

The size and weight difference results from the nature of accordion reeds, which produce sound when air is moved through them in one direction only.

(On multi-row fourth-apart instruments, players can to some extent counter the natural push-pull effect with a row-crossing playing style that "smooths out" the musical phrasing; on semitone-apart systems, depending on the key of the piece being played, players may be obliged to adopt a smoother style.)