From classical to classic rock and pop, musical theater and opera, Chicago a cappella shines a spotlight on your favorite songwriting and performing duos from, well, the past few hundred years or so. Where else could you find Mozart and Da Ponte, Stan Getz and Astrud Gilberto, Elton John and Bernie Taupin, Gilbert and Sullivan, Simon and Garfunkel, George and Ira Gershwin, and the Captain and Tennille all in the same room?