A concertina is a musical instrument invented by Professor Wheatstone, the principle of which is similar to that of the accordion. It is composed of a bellows, with two faces or ends, generally polygonal in shape, on which are placed the various stops or studs, by the action of which air is admitted to the free metallic reeds which produce the sounds. In the English concertina the compass is three octaves and three notes. The German concertina is generally considered an inferior instrument, Research Concertina
In music, fingering is (a) the art of dexterously applying the fingers to a musical instrument in playing, (b) The marking of the notes of a piece of music, as for the piano, organ, harmonium, concertina, etc, so as to guide the fingers in playing. Research Fingering