A diatonic button accordion. The melody-side keyboard contains one or more rows of buttons, with each row producing the notes of a single diatonic scale - i.e. when a button is depressed, a different note is produced when pushing the bellow than when pulling the bellow. The buttons on the bass-side keyboard are most commonly arranged in pairs, with one button of a pair sounding the fundamental of a chord and the other the corresponding major triad (or, sometimes, a minor triad).

Diatonic button accordions are popular in many countries, and used mainly for playing popular music and traditional folk music, and modern offshoots of these genres.

The name melodeon is generally used for any diatonic button accordion in the UK and Australia, but in Ireland is reserved for instruments with a single row of melody buttons and in North America is historically applied to various 19th century reed organs.