The Richter-tuned harmonica, or 10-hole harmonica (in Asia) or blues harp (in America), is the most widely known type of harmonica. It is a variety of diatonic harmonica, with ten holes which offer the player 19 notes (10 holes times a draw and a blow for each hole minus one repeated note) in a three-octave range.
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The standard diatonic harmonica is designed to allow a player to play chords and melody in a single key. Because they are only designed to be played in a single key at a time, diatonic harmonicas are available in all keys. Harps labeled G through B start (on hole 1 blow) below middle C, while Harps labeled D♭ through F♯ start above middle C (C4). Here is the layout for a standard diatonic harmonica, labeled C, starting on middle C (C4).
Although there are three octaves between 1 and 10 "blow", there is only one full major scale available on the harmonica, between holes 4 and 7. The lower holes are designed around the tonic (C major) and dominant (G major) chords, allowing a player to play these chords underneath a melody by blocking or unblocking the lower holes with the tongue. The most important notes (the tonic triad C–E–G) are given the blow, and the secondary notes (B–D–F–A), the draw.