The accordion is a hand-held organ with a central bellows that the player pushes and pulls to produce the air supply. For this reason, the accordion and other related instruments are often referred to as squeeze boxes. The accordion is also classed as a free-reed instrument. The reeds are made of steel and produce the sound when vibrated by the air. Treble reeds are mounted inside the right side of the instrument and are controlled by the player’s right hand. The bass reeds are mounted inside the left side of the instrument and controlled by the left hand. The designation “free” means that the reeds are mounted on a common reed block rather than inside a pipe. The accordion is an exceptional instrument in that it has legitimacy in classical, folk, jazz and popular styles. It is also an instrument has been improved in recent years as more recent designs have decreased the weight of the instrument, and most recently there have been MIDI adaptations.
The invention of the accordion is routinely credited to Europe where Cyrillus Demian obtained a patent for the instrument in Vienna in 1829. Nevertheless, there were a number of antecedents for free-reed instruments. The idea of a portable, walking-around keyboard instrument seems to have captured the 19th-century imagination. Demian’s original contributions seem to have been the name, and the provision of left hand buttons to play chords.

Among earlier free-reed instruments, the Chinese “sheng,” a mouth-blown instrument, which Europeans first encountered when one was brought to Russia in 1770s, is always mentioned in relation to the origin of the accordion. The Baroque regal, a specialized pipeless organ that used oboe-like, cane reeds (rather than steel reeds), also employed the free-reed concept, although a second person was needed to pump the bellows. The harmonium, originating in Paris in the 1840s, was a free-reed console instrument, essentially a pipeless organ with a foot-operated bellows. The harmonium was used in homes until electronic organs superceded them in the 1930s. Other 19th-century hand-held instruments to use the principal of free reeds include the concertina (patented in 1829 in Britain), the bandoneon (originating in Germany around 1840 but becoming popular with Argentine tango bands), and the melodeon (which is also known as the German accordion). The bandoneon is an adaptation of the concertina with a different fingering system. Unlike the accordion, the concertina only includes treble notes, and the available notes are divided between the two hands. The hand-held harmonica is a blown free-reed instrument.

Early accordions, such as the German melodeon, were “single action.” Single-action instruments have a row of bottons for the right hand, and each button produces two notes, one note for a push of the bellows and a different note for a pull of the bellows. For example, when the E button is pressed on closing the bellows, an F willed sound when the bellows are opened. This single-action arrangement means that the four fingers of the right-hand control eight notes, or an octave. Given the push and pull needed for each note, the single-action melodeon produces a very rhythmical effect, and for that reason it is very suitable for dance music.

The piano accordion with its piano-like keyboard for the right hand must be a double action instrument. The instrument comes in a variety of sizes, which are defined by the number of buttons for the bass notes. Usually there are at least 32 bass buttons, but a full-sized model will have 120 left-hand buttons arranged in six rows, as well as a right-hand keyboard of 3½ octaves. There are many different arrangements and functions for the bass-note buttons. In a Stradella bass system, the buttons produced chords, but in a Free Bass system the buttons are single notes. The free bass system is used in classical music and on concertinas and bandoneons.