Anglo concertinas play a different note on the push and pull of the bellows. Lower notes are on the left-hand end; higher notes, on the right-hand end.
20-Button Anglo Concertina
A 20-button Anglo concertina has 2 rows of buttons, 10 on each end. Each row is in a different key, most often C and G, although the lower-pitched combination of G and D is also common. A 20-button concertina can play nicely in its home keys and their relative minors. If your vision is playing tunes on your front porch or by your campfire and you’re not concerned about playing with other instruments, a 20-button concertina might work for you just fine.
30-Button Anglo Concertina
A 30-button concertina adds a row of accidentals, so while it isn’t completely chromatic throughout its range, it will play comfortably in several keys beyond the home keys. If you will be playing with others or think you might someday, a 30-button concertina is more suitable than a 20-button model.
A Word on Custom Layouts
Although radical changes to the tried and true are not always feasible and not typically advisable, some modifications are fairly common. For instance, people getting instruments made to order with the Wheatstone layout sometimes replace the d# on the push on the first button of the right-hand accidental row with a c#, thereby gaining a c# in both directions on a button that’s easy to reach, and in a useful range–the octave above middle c–too.
R. Morse & Co. Ceili Anglo Concertina, inside and out.