Opera numbers may be arias, but also ensemble pieces, such as duets, trios, quartets, quintets, sextets or choruses.
[1] The replacement of numbers with more continuous music began in operas by Jommelli, Traetta, Gluck, and especially Mozart, whose late operas Le Nozze di Figaro and Don Giovanni contain several segments in which different numbers are unified by bridge passages to form a musical whole.
[2] The number opera was strongly condemned by Wagner for dramatic reasons, and he replaced it with continuous music that advances the drama without interruption.
[2] However, in the 20th century some composers intentionally revived or adapted the number opera format, e. g., Busoni's Arlecchino (1917),[3][4] Berg's Wozzeck (1925),[1] Hindemith's Cardillac (1926, rev.
[2] In operetta and in popular music theatre, number opera format has remained the norm.