Blues harp or cross harp denotes a playing technique that originated in the blues music culture, and refers to the diatonic harmonica itself, since this is the kind that is most commonly used to play blues. The “classical” harmonica for blues playing was the Hohner Marine Band, which was affordable and easily obtainable in various keys even in the rural American South, and since its reeds could be “bent” (see below) without deteriorating at a too rapid rate.
A diatonic harmonica is designed to ease playing in one diatonic scale. Here is a standard diatonic harmonica’s layout in the key of C (1 blow is middle C):
Blues harp subverts the intention of this design with what is “perhaps the most striking example in all music of a thoroughly idiomatic technique that flatly contradicts everything that the instrument was designed for” (van der Merwe  p66), by making the drawn notes the primary ones . Since these “draw” notes are more easily bent (for holes 1-6) and consist of (relative to the key of the hamonica) II, V, VII, IV, VI. In the case of a major-C harmonica, this will be D, G, B, F, A. This allows two things:
- Bending on the draw notes; in the case of C-keyed harp, this will let with B and D bending down and F up (G down); These basically created a blues notes.
- An approximation of the blues scale, which consist of I, III♭, IV, V♭, V, VII♭. If played on a C-keyed harp, this produces a blue scale in G: G, B♭, C, D♭, D, F. (This resembles the tuning of the bottleneck guitar).
The player can play slurs or bends around the minor/major third of the scale and around the tritone/fifth of the scale, both of which are vital to many blues compositions. For a further discussion of “bending” on the harmonica, see the harmonica article.
The key played in this style is one fifth above the nominal tuning of the harmonica, e.g. a C harmonica is played in the key of G. Therefore, to be in tune with a normal guitar tuning of E, an A harmonica is often used. This is because by playing the C harmonica in G, or A harmonica in E, the dominant or seventh chordis produced in place of the tonic chord, and in the blues, all chords are typically played as dominant (seventh or ninth) chords.
This is playing in second position, called “cross harp.”
If one used solo-tuned harmonica instead of richter tune, it will be in the 3rd position, a ii-minor key. So in the case of C harmonica, it will be in the key of D minor. This is called “slant harp.” Minor keys can also be easily played in 4th and 5th positions.