Accordions come in many different styles and key-note systems. There are several accordion types and sizes. Most accordions have two parts connected by the bellows : the right side (the melody side) and the left side (the bass side). On the right they have the treble keys used to play the melody lines. On the left accordions have the bass key and bass chord keys. Generic types of accordions, along with their specifications.

1) Piano Accordion
The most popular accordion is the piano accordion. On the right side it has piano keys usualy 25 to 41 keys and on the left side has the bass keyboard which is usually the Stradella system(12 to 120 keys), or one of the various free bass systems.

2) Diatonic Accordion
There are many variations of the diatonic accordion. Generally there are buttons for the left hand and for the right hand. The significant distinction for the diatonic accordion is that for a single key the pitch of the note changes as the bellows is pushed or pulled.

3) Chromatic & Bayan Accordions
These accordions have buttons for both the right hand (treble side) and the left hand (bass side). The arrangement of the bass buttons is similar to the arrangement for the bass buttons for the piano accordion, i.e., either "stradella" or "free bass". For the right hand the button configuration has either 3, 4, or 5 rows of buttons.

4) Concertina
Concertina is another free-reed musical instrument. It has a bellows and buttons typically on both ends of it on the sides. It is usually much smaller the a regular piano accordion and each button produces one note, while accordions typically can produce chords with a single button.

Chromatic accordions are popular among musicians in Europe. It played a big role in the development of French, Scottish and English folk music. Two main types of chromatic button accordions exist, the continental and the British chromatic accordion. Continental chromatic accordions have the greatest range of treble notes. The B system is better for more technically challenging works, and the C system is easier to play chords. British Chromatic Button Accordions, popularized by Scottish accordionist Jimmy Shand, have three rows of buttons tuned in the keys of B, C and C#. The instrument has played a significant role in Scottish, English and Irish traditional music.

Learn more at this website:
http://www.accordion-fanatic.com/chromatic-accordion.html