A button accordion is a type of accordion on which the melody-side keyboard consists of a series of buttons. This differs from the piano accordion, which has piano-style keys. Erich von Hornbostel and Curt Sachs categorize it as a free reed aerophone in their classification of instruments, published in 1914. The sound from the instrument is produced by the vibration of air in reeds. Button accordions of various types are particularly common in European countries and countries where European people settled. The button accordion is often confused with the concertina; the button accordion's buttons are on the front of the instrument, where as the concertina's are on the sides and pushed in parallel with the bellows. Main components All accordions and concertinas have three main components: the reeds, bellows, and buttons or keys. Pushing or pulling the bellows slower or faster makes the sound softer or louder, respectively. The accordion has free reeds on both the treble and bass sides. In modern accordions, the free reeds are generally made of tempered steel. The press of a button or key opens a valve to allow air to pass through the reed or reeds to make a sound when the bellows are pumped in or out. In the diatonic button accordion, reeds are fixed in pairs so that one note sounds when air moves in, and a different one when air moves out. The button accordion has melodic notes on one side of the bellows (usually the right side), and bass accompaniment notes on the other side (generally the left). Some button accordions have 'stops', which change the tone and are called things like "Organ" or "Trumpet" or "Tremolo". These allow the instrument to produce different tones for a variety of situations. Some popular examples are the three-stop accordion, with two sets of tenor reeds and one bass set, and the classic German four-stop. The three-stop accordion has two sets of tenor and one set of bass reeds. The German four-stop is preferred by Cajun musicians and has one bass, one piccolo, and two tenor stops. This gives the instrument a denser sound. Most diatonic instruments lack switches, though there are some made by companies such as Hohner, as well as the one-row 'Cajun'-type boxes which have usually 3 or 4 stops on top of the box as switches (making it even more akin to a pipe organ), but it is generally