"Club" Melodeons / Accordions and "De-clubbing"The keyboard layout of a "Club" accordion is very similar to that of a conventional diatonic melodeon but has an inner "helper" row of accidentals and a central , sometimes serrated , button that plays the same( unisonic) "Gleichton" in both directions of the bellows.
Sellers sometimes offer the vague explanation that the duplicated note note somehow increases the chord "options" available. More exactly it enables the major chords to be played on both rows in either bellows direction .
Along with the club layout came several "play by numbers" systems involving special notation that required a single note at the centre of the keyboard to correspond with the middle stave of the "music"
De-clubbing simply means retuning the central reeds to diatonic, thus restoring the "missing" note to give a virtually conventional layout . However, the accidentals usually found on the "No. 1" buttons of a 21 button instrument have to be played on the helper row and one bass/chord will be different.