The term ‘harmonica’ has been used for various musical instruments at different times. Today, it primarily refers to the mouth-organ, a small wind instrument invented in 1830s. The player holds the instrument agains their lips and blows into small holes, behind which are a set of metal reeds, one for each note. Each hole on the harmonica can produce two different notes, one when the player blows into a hole, and another when the player draws air through the hole. The harmonica is moved side to side across the mouth by the player to play different notes.
The two main types of harmonica today are the ‘diatonic’ or ‘blues harmonica’, and the ‘chromatic’. Without ‘bending’ the notes, the blues harmonica can only play certain notes of a diatonic scale that it is tuned to. The chromatic harmonica on the other hand has a slider activated by a button on the side of the instrument which allows air to be directed to a different set of reeds. This allows the player to play all twelve notes of the chromatic scale.