With a body that's typically constructed of wood or plastic and a metal cover plate, the harmonica operates by a set of reeds that vibrate when you blow or suck air through any of the 10 holes. Blowing creates one pitch (in a diatonic harmonica 1, 3 and 5 of the scale), while drawing air through the same hole creates another pitch. Diatonic harmonicas are designed in a specific key, while chromatic harmonicas include every natural and sharp note in a chromatic scale, allowing their players to modulate back and forth between keys. Chromatic harmonicas also include a slide button that helps to operate sharps and flats.
According to some sources, the harmonica is derived from the ancient Chinese sheng, which was made of a gourd and reeds. A German teenager named Christian Friedrich Buschmann held the first patent for a harmonica-like instrument, which he called an "Aura." In the mid-1800s, an American immigrant named Richter developed the first diatonic harmonica, after which all diatonic harmonicas since have been modeled.
It wasn't until the late 19th century, though, that Mathias Hohner—a German clock maker—developed what we now know as a harmonica. Hohner exported several of them to his cousins in America, who then, in turn, hocked them to aspiring musicians far and wide. Nowadays, although other harmonica manufacturers are successful, none have been quite as successful as Hohner harmonicas, which are often considered the most reliable such instruments. Used in jazz, blues, rock, country and folk music, Harmonica is one of the most diverse (and deceptively difficult) instruments around.