" if yer gonna play that C harp on a rack and play yer geetar too you are gonna need to..."

Well, you'd play in G if you want to (and are able to) play "cross-harp" or blues style ~ or, in C to play straight-ahead majoy-key style.

As a non-player, I started out assuming that harp-in-a-rack, as per Woody, Bobby, Ramblin' Jack, et. al., was always cross-key blues harp, but I've been told otherwise and eventually come to believe I was probaly wrong. For a given song, those guys might choose to play either way.