Duet concertinas have buttons arranged in 5 columns (Crane) or 6 columns (MacCann). There are also Jeffries and Hayden Duet systems, but both are vanishingly rare. All duet systems play the same note on push and pull strokes.

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Prices will range from £600 to over £3000 - the basic rule is the more buttons the better. As always, the 8-sided Wheatstone Aeola is probably the most popular and the most expensive - and rare finishes will make them even more so.