The button layout is altered so that the high notes are played on the right and the low notes on the left. This makes it easier to play a melody with an accompaniment. There are several keyboard systems available: * MacCann duet * Crane duet * Triumph duet * Jeffries duet * Hayden duet * Wheatstone systems: * Wheatstone Double * Wheatstone Duett * Wheatstone Chidley The Hayden system is the most recent development, invented by Brian Hayden.