A concertina, like the various accordions, is a member of the free-reed family of instruments. It was invented in 1844 by Sir Charles Wheatstone. Concertinas typically have buttons on both ends and are distinguished from an accordion (piano or button) by the direction of their button travel when pushed. Concertina buttons travel in the same direction as the bellows whereas accordion buttons travel perpendicular to the direction of the bellows.

The name Concertina refers to a family of hand-held bellows-drive free reed instruments constructed according to various systems. The systems differ from one another:

in the notes and ranges available;
in the positioning of the keys (buttons);
in the sonoricity of the notes provided by the keys:
the keys of the bisonoric instruments produce differing notes on the press and on the draw;
the keys of the unisonoric instruments produce the same note on the press and on the draw;
in the ability to produce sound in both bellows directions:
single action, producing sound only in one bellows direction (usually found only on bass instruments);
double action, producing sound in both bellows directions;
in size and shape of the instrument and the technique required to hold the instrument.
To a player proficient in one of these systems, a concertina constructed according to a different system may be quite unfamiliar.

The most common concertina systems are listed below. The list is not exhaustive, as the concertina is not only a venerable and widespread instrument, but also an evolving instrument: modern experiments in concertina construction include chromatic scales offering more than 12 steps per octave, and instruments which allow the pitch of the notes to be sharped or flatted by the performer.

Chemnitzer concertina and other German concertinas
There are various German concertina systems which share common construction features and core button layout. In the United States, particularly in the Midwest, the term "Concertina" often refers to the Chemnitzer concertina. Chemnitzer Concertinas are bisonoric (see above) and are closely related to the bandoneón, but with a somewhat different keyboard layout and decorative style, with some mechanical innovations pioneered by German-American instrument builder and inventor Otto Schlicht.

Bandonion or bandoneón
Of special note is the bandonion or bandoneón, a German concertina system the bisonoric layout of which was devised by Heinrich Band.

---------------------------------------