Cross-harp? It's because if you try to play it in the same key as the guitar player, he'll become quite cross with you. Or you'll get all crossed up trying to remember whatthehell key you're really in. HellifIknow. Let me see if I can 'splain this without getting you further crossed up. Your basic 10-hole diatonic harp has its key defined by the note that is produced when you blow on the first (lowest) hole on the harp. As you blow and draw on successive holes going across the harp, you'll create (more or less) a major scale. BUT, if you start by drawing in on the second hole and using that as your key defining note (or, let's say the Do, as in Do, Re, Mi), you'll be able to access lots of accidentals and blue notes that weren't available when playing "straight" harp. That second hole drawn note is five notes up the major scale from the first hole, blown note. That is why you use an A harp to Play in E. etc. Because you're starting from a different point of reference than whatever is stamped on the top plate of the harp. But WHY???? Because there's more important notes that can be easily "bent" available to you when playing cross harp, not to mention blue notes, etc. Because it's just more awesome. And Brootalz Special 20 for me too, after decades of Blues Harps. In about 1970 I was working and teaching in a music store and I'd get the question all the time: "What's the difference between a Marine Band and a Blues Harp?" I'd answer "About 50 cents".