The predominance of music or text is one of the most reliable characteristics when trying to distinguish an opera from a musical. Musicals use spoken text more regularly between sung numbers. In a musical, the text is the major element in terms of creating drama. In comparison, opera relies more on musical numbers to move the dramatic action forward. Some operas contain portions of spoken text, but the music takes precedence as the main conduit of the drama.
Operas and musicals come out of the same tradition of singing onstage to convey a story or drama. The first operas appeared in Italy at the end of the 16th century. They were stage works that sought the use of music (vocal and instrumental), acted stage drama, dance and scenery to convey various ideals and stories. Writers and composers frequently preferred ancient or Greco-Roman subjects. Opera traditions and styles further developed. Musicals, however, are a relative newcomer, dating to the mid-19th century. They are an outgrowth of comedic theatrical presentations, circuses and vaudeville acts whose intention was to provide entertainment to a large public while incorporating music.
Operas and musicals today have a noticeable difference in singing style. Operatic voices are generally characterized by a roundness and use of vibrato. Vocal style for musicals tends to more resemble popular music. Both hold clear diction (good vocal pronunciation) as an ideal, yet singing in musical theater more resembles day-to-day speech.
Early operas were accompanied by various instrumental ensembles consisting of stringed and wind instruments, as well as keyboard and percussion instruments at times. As musicals developed in the 19th century, the instrumentation of their accompaniment remained relatively similar to that of operas. Only in the 20th century did musicals begin to economize their instrumentation, making use of developments in technology such as microphones and speakers. This change reflected parallel changes in popular music. Contemporary operas have also experimented with instrumentation, at times influenced by popular music but also simply seeking new forms of musical artistic expression.