Richter-tuned harmonica
The Richter-tuned harmonica, 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.
The standard diatonic harmonica is designed to enable playing chords and melody in a single key. Because of this design, playing in different keys requires the player to have a separate instrument for each key they play in. Harps labeled G, A♭, A, B♭ or B start (on hole 1 blow) below middle C, while those 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 is a three-octave distance between 1 and 10 "blow", there is only one full major scale available on the harmonica, using holes 4 through 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 notes most important in the key (the tonic triad C–E–G) play during the blow, and the secondary notes (B–D–F–A) are on the draw.