The music needs room to breathe-to repeat itself, to develop instrumentally and harmonically, and to change pace and color as the situation demands. Since singing diminishes the intelligibility of a text, the words themselves must not be too hard to follow. Opera, then, must forgo a good play's verbal richness and precision, but in its music it gains the richness of a language that speaks directly to the emotions. In writing music that will serve the dramatic needs of the text, most composers employ certain conventions, such as writing for the higher registers of a voice or instrument to indicate passion, or employing dissonant harmonies to depict fear. These conventions are not wholly arbitrary, however.
Voices do rise when people become excited, and the physical sensation of fear is inharmonious.
A skilled opera composer will use still more subtle techniques to enhance the dramatic effect of the music. The melodic line must be suited to the words it is meant to carry, and the harmony must convey the ebb and flow of feeling and action. Rhythms vary for impetuous declamation, stately ceremonial ensembles, love duets, or arias. Many great composers, such as Christoph Willibald Gluck and Richard Strauss of Germany, Claude Debussy of France, and Alban Berg of Austria, have emphasized the dramatic in their operas. The Opera Repertory Today's typical opera repertory mainly comprises works from the 19th century, plus a few from the late 18th and early 20th centuries.