Agree with John. "Primarily" means nothing-early rock had a lot of piano in it, technically an acoustic instrument, bass in the early days was mostly stand up bass, again acoustic, and the same goes for horns. Someone like, off the top of my head, Pete Townshend (when he plays Who material solo) is still playing loud, and at times aggressive, which to me is rock regardless of being played on acoustic. Now there are lots of other examples of this, but I think the argument is not one of instrumentation per se, but of attitude. I saw John Hiatt about 10 years ago with his band, and although he was playing acoustic, that was one of the loudest damn shows I have ever been to, and was definitely rock in terms of attitude. Flip side is when I saw Nils Lofgren do a show at a little club, which was a lot more mellow, but the attitude, was all rock. In both cases, neither of these guys qualifies as singer songwriters (Ok, Hiatt, maybe some would argue), or blues, or country or whatever. That to me means acoustic rock. Bob Seger's Night Moves is another classic example, strummed acoustic, bass, drums, piano, female backup vocalists. Not a loud song, but no question, he's rock, and in the case of that song, an acoustic rock song if you will