What is a Harmonica cross harp?

Answer 1

In blues harmonica "Cross-harp" refers to the practice of using a harmonica which is in a different key than the rest of the song. This is actually necessary (for reasons beyond my understanding) as a C harmonica will not sound right playing along with a band whos playing a song in the key of C. If the rest of the band is playing in C, get out your F harp.

Likewise if the band is playing in A, you need your D harp.

Wikipedia puts it thusly...

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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 chord is 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." ----------------------------------------------------------------------- Which isn't terribly helpful, but I have found a chart which is... it is linked below.

Essentially every harmonica is a scale of the key its in. If you have the proper cross-harp for the key the blues song is in, nearly every note on that harp will fit anywhere you care to put it... you can get by with an off-key harp, but you will have a lot of foul notes to step over.

And the way I have remembered it is this. If youre a guitar player, put your finger on an A on the fretboard, now move that finger one string down to D... D is your cross-harp! Put your finger on a G, move your finger down one string & C is your cross-harp!