The accordion is a wonderfully versatile musical instrument. It is used in nearly all types of music, from the obvious polka to zydeco to jazz and even rock. Two great examples of the latter are They Might Be Giants and "Weird Al" Yankovic.

The accordion is a reed instrument, and consists of two halves, seperated by a bellows, like so:

+---+\/\/\/\/\/+---+
|   | | |  | | |   |
|   | | |  | | |   |
|   | | |  | | |   |
+---+/\/\/\/\/\+---+

Each side contains a bank of metal reeds. Air is allowed through any given reed by pressing a key or button which opens a valve covering the reed. Some accordions have buttons on both sides, these are usually called concertinas. Sometimes, an accordion will operate like a harmonica in that one button will produce a different sound when the bellows are drawn than when they are closed, but most produce the same sound in either direction.

The accordion we will deal with here is the common piano accordion. The (player's) right side is outfitted with a piano keyboard for melody, while the left side is outfitted with a large (intimidating for the beginner) array of buttons for bass and accompaniment. The size of the array ranges from 6x1 to 20x6, and even larger sizes exist. The smallest complete size is 12x6, giving 72 buttons. This size is arranged like so:

         BELLOWS

  Db  Eb  F   G   A   B
  | Ab| Bb| C | D | E | F#
  | | | | | | | | | | | |
B O O O O O O O O O O O O  3rd       T
O  O O O O O 0 O O O O O O  Root     O
T   O O O O O O O O O O O O  Major   P
T    O O O O O O O O O O O O  Minor
O     O O O O O O O O O O O O  Dominant 7th
M      O O O O O O O O O O O O  Diminished

          HAND

Note that the "Root" button in the C column is usually marked with a depression, rhinestone, or other finger-aiding device, not unlike those little marks on computer keyboards.

The root note of each column goes up a perfect fifth in the upwards direction and down a perfect fifth in the downwards direction. The "Root" and "3rd" buttons play the root and major third of their respective columns. The "Major" and "Minor" buttons play major and minor triads starting on their column's root note. The "Dominant 7th" and "Diminished" buttons each play a diminished triad respectively starting on the root and major third of their column.

Bigger or smaller accordions will add or remove columns from either end, and smaller accordions will remove rows in the following order: Diminished, Dominant 7th, Minor, Major, 3rd.

Note that this configuration allows for a major or minor arpeggio to be played easily; the three buttons involved are nicely juxtaposed and readily available right above the major chord button. Major, minor, and other modal scales can be played with similar ease. Playing any permutation of the I-IV-V movement is easy, the three columns involved are are right next to each other.

Aside from pumping the bellows, learning the keys, and mastering the buttons, the only major accordion technique is what is known as the bellows shake. This consists of rapidly pumping the bellows in and out to create a string of stacatto notes, not unlike double tounging used on brass instruments.

Most modern accordions contain multiple banks of reeds, always on the right side, often on the left side. Tabs situated on either side above the keyboard and buttons allow selection of reed banks. Accordions will usually have from two to four banks of reeds on the right side, and up to about eight on the left.

The reed banks on the right are classified as either 16', 8', or 4' (this is the same manner in which organ pipes are classified). 8' is midrange, 16' is an octave lower and 4' is an octave higher. Accordions have always one or two 8' reed banks, and usually a 16', 4', or both. Combinations of these banks are selected by the tabs above the keyboard, labelled with a symbol denoting which banks they activate:

  _____
 /_____\
|__*_*__|
 \_____/

Here, the two dots in the middle indicate that two 8' reed banks are activated. A dot in the top represents the 4' reed bank, and a dot in the bottom represents the 16' reed bank. Most accordions having two 8' banks will have one detuned slightly in order to create a tremolo effect. The tabs also have names denoting the type of sound which they produce:
                               Master
  Bass     Bandon     Organ    or Full
  _____     _____     _____     _____
 /_____\   /_____\   /__*__\   /__*__\
|_______| |___*___| |_______| |__*_*__|
 \__*__/   \__*__/   \__*__/   \__*__/

           Violin or
 Clarinet   Musette    Oboe     Piccolo
   _____     _____     _____     _____
  /_____\   /_____\   /__*__\   /__*__\
 |___*___| |__*_*__| |___*___| |_______|
  \_____/   \_____/   \_____/   \_____/

Bass (left side) reeds work similarly, although the system is a bit more complex. One does not switch bass reed banks very often; this is why many accordions come without switchable banks on this side.

Not all accordions use reeds. Some are fully electronic, operating not unlike a MIDI keyboard, and not requiring any bellows pumping. Accordions also come with built-in microphones or can have some added for playing through an amplifier.

Well, I think that's all I can ramble on about with regards to accordions. If you are interested in learning how to play, buy a used accordion (these go from $100 to $1000) and a teach-yourself book (instructors are hard to come by). If you cannot find any good used accordions, new ones aren't all that expensive either, ranging from $300 to $3000 dollars (super-magical five-digit priced models also exist). If you can find a lesson teacher, good for you, take lessons from him/her!

I hope this answers all the questions you never even thought to ask about accordions. If you have more, feel free to /msg me.