Opera arose from several different sources, among them the revival of Roman comedy in the late 1400s, mostly intended for entertainment at various ruling courts in Italy. It didn’t take long for rulers to see political and diplomatic advantages in mounting spectacular performances of them, and by the middle of the 1500s, they routinely mounted comedies with musical interludes between the acts. These interludes, intermedii in Italian, grew to become dramatic spectacles in their own right, involving the musical talents of the entire court establishment. Most Italian courts of the time boasted excellent trombonists.
The music-loving Medici family managed to persuade the Emperor to name them as hereditary dukes, but unlike most other dukes, their background was in banking and commerce, not military strength. They had the greater need than other ruling families for music and drama to project their strength and stability, so while other courts mounted comedies, by now newly written, with intermedii, the Florentines presented more of them and documented them more carefully. For two Florentine spectacles, all of the music has survived. For the rest, elaborately printed commemorative books describe them in enough detail that we can know exactly what musical numbers were composed for the occasion and exactly what voices and instruments performed each one.
Scenes of the intermedii could take place anywhere from the heavens (or Mount Olympus, or otherwise seat of divine power) to a variety of stereotypical earthly locations to the underworld. Certain instruments were conventionally considered especially suitable for each setting. Scenes among the gods (always immediately recognizable as standing for the hosts ) used all of the instruments typically played by courtiers and all of the instruments played publicly on state occasions. The latter specifically means the court wind band, comprising cornetts (or shawms) and trombones. The illustration shows the demons lamenting that with the coming Golden Age, to be ushered in by the 1589 wedding being celebrated, there will no longer be souls for them to torment. Sorry I couldn’t find a larger image, but the trombonists would have been out of sight, anyway.
Trombones occasionally participated in the various earthy scenes (both on land–especially the countryside–and sea) occasionally, but scenes in the underworld typically required four of them, playing in their lower register. Monteverdi’s Orfeo, both a court spectacle and an opera, written in a transitional figure, illustrates these standard conventions nicely. Earthly scenes and underworld scenes use completely different instrumentation. Strings and recorders play the pastoral scenes; cornetts and trombones play the underworld scenes.
For the most part, however, opera arose as a reaction against the pretentiousness and dramatic absurdity of the intermedii. The mixed string and wind ensembles that developed by the end of the 1500s must have sounded spectacular on occasions where there was plenty of time for rehearsal. Otherwise, inherent intonation problems rendered them intolerable to many listeners when presented again in more ordinary circumstances. Except for Orfeo and possibly some of Monteverdi’s lost works, the earliest operas used only a string accompaniment.
After the first commercial opera theater opened in 1637, no opera offered to the public included trombones in the orchestra until 1762, when Gluck used them in Orfeo ed Euridice. Courts continued to present ostentatious extravaganzas on special occasions, and many of these used trombones. All of the music exists for the longest and most complicated of them, Antonio Cesti’s Il pomo d’oro, the Spiderman musical of its day in terms of complicated machinery and production delays. It kept the trombonists as busy as anyone else.