Hohner developed the Club system to allow diatonic players the ability to play chromatically for "club" gigs. It's based on the 2 row international (Vienna) system of home row (tonic) and companion row (fourth) with two significant developments. 1) The companion row contains a uni-sonoric home note "gleichton" that facilitates playing in the home key in both directions 2) The additional helper row allows the player to play chromatically in both directions. The modernized steirische models extend the concept of gleichton across multiple rows. The extra row of accidentals or reversals can be found on various international and semi-tone layouts to play in additional keys or facilitate fingering.