We're having a debate at the BWTB page over which is the correct key of harp need to play crossharp. As I understand it you take the key of a song, say A, and the 4th note up the major scale is your crossharp key, which would be D in this case.
Look at this chart: http://harmonicajoe.com/cross-harp-chart/
Others are saying it's the 5th. I'm not a big time harp player but the rule of the 4th works for me if I want to play blues style harp. Is this where the confusion is happening..blues versus other styles?
A real deal harp player needs to chime in and educate us here.