Incidental music is music which has been written for a play. It is not like opera where there is usually music all the way through. Incidental music will be used between scenes or for particularly important moments in the play or for songs sung by the actors. It is called “incidental” because the music is not as important as the play.
Incidental music was used as long ago as the time of Ancient Greece. It was used in the 16th and 17th centuries, especially in the plays of Shakespeare whose characters often sing songs. At this time incidental music was used more often for comedies than for tragedies. Tragedies were about important people who were busy ruling their country and having wars. In the plays they spoke in verse and were too noble to sing. People thought that comedies were more suitable for including music. The songs were often sung by actors playing the parts of shepherds, nymphs or clowns.
In later centuries music was used for all types of serious drama. Sometimes large orchestras were used. Many famous composers wrote music to be played by an orchestra during plays. A famous example is the incidental music for Shakespeare’s “A Midsummer Night's Dream” composed by Felix Mendelssohn in 1842. Other composers worked with living poets such as Goethe and Schiller. Beethoven wrote music for Goethe’s play Egmont. This music is very closely linked to what happens in the story: the overture describes the harshness of the Spanish oppression of the Netherlands which is what the play is about. Schubert’s famous Rosamunde overture comes from a play called Die Zauberharfe (The Magic Harp). Schumann’s music for Manfred is different: it was never intended to be performed with the play, it was just inspired by Schumann’s imagination. Many other composers wrote incidental music, e.g. Sibelius (for Maeterlinck’s play Pelléas et Mélisande) and Grieg (for Ibsen’s play Peer Gynt).
In all these examples the music is now usually heard at concerts, not as part of the original play. Most of them have overtures (music played before the play starts) and these overtures are often performed separately, very often at the start of concerts.
Incidental music was not used so much during the 20th century although the plays of Brecht, which are often about political propaganda, needed a new kind of popular music. Brecht worked with composers like Weill and Eisler who understood the kind of message that he wanted in his plays. Their music for Brecht’s plays is similar to cabaret music.
Nowadays electronic music is often used in plays.