Musicians who work professionally with ballet and dance companies sometimes wonder if they haven't entered a foreign country--a place where the language and customs seem so utterly familiar and so bafflingly strange at the same. To someone without a dance background, phrases and terms--boy's variation, pas d'action, apotheose--simply don't fit their standard musical vocabulary. Even a familiar term like adagio means something quite different in the world of dance. Like any working professional, those conductors, composers, rehearsal pianists, instrumentalists and even music librarians working...