Okay, I'll try again - I gave a magnificent explanation earlier and then - you'll never guess what - my computer crashed as I was trying to send the message ...

The term "straight harp" - known now among aficionados as "first position" - indicates that the harmonica is being played in the key for which it was designed; e.g., a C harp is playing a melody in the key of C. The term "cross harp" indicates that the harmonica is being played in the key two-and-a-half tones down from the key the harmonica is designed for; e.g., a C harp playing a melody (or more often an improvisation) in the key of G. If you think of the I-IV-V pattern, the cross-harp is in the key of IV.

There are other positions used by advanced players, which presumably is the reason why the folksy terms "straight", "cross" and "slant" (third position) are being pushed aside in favour of the more clinical, numerical designations.

Cross harp is the usual position used in blues and blues-influenced music.