Concertina
To get this section started, I'm going to borrow a chunk of text from my site the Concertina FAQ.
The concertina belongs to a class of instruments known as Free Reed instruments, which also includes accordions and harmonicas. It was developed in 1829 and 1830 by Sir Charles Wheatstone of Wheatstone bridge fame after several years of building prototypes, a few of which still exist (in 1829 he patented its direct predecessor, the Symphonium, in a document which also described a very concertina-like instrument, but he did not actually patent the concertina itself until 1844). The already-existing family musical instrument firm of Wheatstone & Co switched over to manufacturing concertinas, each one expensively hand-made by highly skilled craftsmen, and at first the concertina was very much an instrument of the middle and upper class drawing room. Its fully chromatic range was suited to classical pieces, with its fast action lending it to "party pieces" such as The Flight of the Bumble Bee. In due course other firms such as Lachenal and Jeffries were founded (several by ex-Wheatstone employees) the cost of concertinas lowered, and the instrument moved out of the drawing room and into the world of popular music.
It became popular with music hall performers, several of whom, such as Percy Honri (who billed himself as "A concert-in-a turn") and "Professor" J. H. MacCann, were musicians of the highest virtuosity. The Salvation Army liked it for its portability and strident tone. Concertina bands were formed, playing marches and other popular pieces (and commemorated to this day by the Concertina Brewery, who brew in the cellar of the old Mexborough Concertina Band Club in South Yorkshire). It also became a favourite of traditional musicians throughout the British Isles.
In the 20th Century the instrument gradually fell out of favour, and one by one the makers closed or went out of business. Wheatstone's themselves (by this time owned by Boosey & Hawkes) closed in 1968, the last survivor being Crabbe & Co of Islington who closed in the late '80s.
What saved the instrument from gradually dwindling away into obscurity, as far as the UK was concerned, was the Folk Revival from the '60s onward. Performers looking for a different sound from the ubiquitous guitar were drawn to the concertina for all its old virtues of versatility and flexibility combined with portability. In addition the concertina permitted song accompaniments that were free of the rhythmic straitjacket that the guitar in unskilled hands tends to impose upon everything. For folk and morris dance the anglo concertina and its accordion cousin the melodeon proved ideal. People started making concertinas again, many of a quality to equal anything made by the old companies.
There are several distinct types of concertina, all sharing the same basic design of folding bellows with buttons at each end, and anything from 6 to 12 sides in cross-section. Where they vary is in the layout and function of the keys. The variation is so great between the types as to effectively render them different types of instruments - the player of one type or "system" will almost certainly not be able to pick up a concertina of a different system and play it without starting almost from scratch to learn it.
Concertinas come in various sizes which govern the range of notes they can play. The most common are treble concertinas. The range of a standard 48 key English concertina is from G below middle C to C 3 octaves above middle C (i.e. the same as a violin). Below them are baritone concertinas which play one full octave below the treble, and the bass which plays one octave lower again. Also fairly common are tenor-trebles which cross the range of the treble and baritone. VERY occasionally you find piccolo concertinas which play one octave above the treble.
The main types are the English concertina, the Anglo concertina and the various systems of Duet concertina.
Concertina players have their own organisation, the International Concertina Association. Other sites worth investigating are the aforementioned Concertina FAQ, Concertina.net, which has a superb and very active forum, and the Concertina Library, a huge and lovingly compiled and organised resource of historical and other documents of relevance to the concertina. All three of these sites have many links to other sites of interest to concertina players.