The accordion was invented by Friedrich Buschmann in 1822 in Berlin. It is a portable wind instrument consisting of two reed organs connected by a folding bellow. Expanding and contracting the bellows provide air to vibrate the reed organs producing the sounds. There is a keyboard on the right side for playing melody notes and buttons on the left to sound bass notes and full chords. The keyboard on the right side of the accordion typically contains 41 keys but the smaller models can contain as few as 25. The full "concert accordion" typically has four sets of reeds called treble shifts, one set tuned in unison, a second set tuned one octave higher, a third tuned one octave lower, and the fourth set, the tremulant, tuned slightly higher than unison. The accordion has a very rich, reedy and organ-like sound. It has the ability to play single or multiple notes on the keyboard (right side) as well as chords on the left side. The accordion has traditionally been used to perform folk or ethnic music, popular music, and transcriptions from the operatic and light-classical music repertoire. Today the instrument is sometimes heard in contemporary pop styles, such as rock, pop-rock, etc and occasionally even in serious classical music concerts, as well as advertisements.