I think it's worth adding that at some point (in the 60's I think[1]) they had a bright idea as to how to include stereo sound in records: one track is encoded in the displacement of the groove parallel to the record, the other in the displacement perpendicular. This meant that old record players could still read one track of stereo records (the parallel one), though given that bands back then went a bit overboard with stereo (a few Beatles' songs have all the vocals right over to one side, for example) that must have been slim consolation.
[1] edit: nope. 1958.
edit no. 2: It seems I was talking a fair bit of crap above. In fact the stereo records were such that a mono player would output the combined left and right channels. See here.