Melodian is another name for a diatonic button accordian. Button accordians have buttons for both the left and right hands of the performer to use; on the melody side, there are one or more rows of buttons, with each row producing the notes of a single diatonic scale. On the other side, 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).