I once worked with an (Irish) sound engineer who was often required to supply pices of music for advertising jingles. He would take a piece of library music and recortd it to quarter inch tape. Since this machine ran at 15 i.p.s. (Inches per second) he could now measure out the required piece with a tape measure. So for a 30 sec commercial he needed 37 feet 6 inches and when the appropriate point was reached, out came the razor blade and, snip, the appropriate lenght of music was handed over to the commercial producers: no regard for musical form or beats.