Blues harp
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.
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.
The valved diatonic is one of the most common ways of playing chromatic scales on diatonic harmonicas. While chromatic is available, valved diatonic is also common, and there are reasons to use a valved diatonic rather than chromatics. It does not have a slide assembly (so that it has less air leakage), and it has a wider tonal range and dynamic. As well, it has a smaller size and is much more suitable to use with microphone, and it is still cheaper than chromatic, even for a premade one like Hohner's Auto Valve or Suzuki Promaster MR-350v.
Valved diatonics are made by fitting windsavers on draw holes 1–6 and blow holes 7–10; this way, all reeds can be bent down a semitone at least, although most players can easily bend down a whole tone. Alternatively, one can simply buy a factory-made valved diatonic such as the Suzuki Promaster Valved.
...
Wikipedia