The ensembles in Mozart's operas are admired -- and rightly so -- for their beauty, their complexity, their formal elegance, their masterful delineation of each character, and for the skill with which conflicts between characters are combined into a musically harmonious whole. But another attribute, no less admirable, and unique to Mozart, is rarely mentioned, if at all. His ensembles are not dramatically static. They do not halt the progress of the play. The action continues during the ensemble, so that the situation at its end is different from that at its beginning,

Verdi, Rigoletto act 3, quartet, Bella figlia dell'amore [Beautiful daughter of Eros]
Mozart, Le Nozze di Figaro act 3, sextet, Riconosci in questo amplesso [With this embrace recognize your mother]

Verdi's quartet does not advance the drama. It only elaborates the situation already established in the preceding recitatives and arias. The Duke and Maddalena are flirting inside the house, while Rigoletto and Gilda, spying on them from outside the house, express their distress at the Duke's licentiousness. This is beautiful music, and beautiful vocal display, and an expert counterpoint of contrasting melodies and conflicting emotions, but it does not move the drama forward. At the end of the quartet, nothing has changed. On the contrary, the drama is at a standstill until the quartet ends.

Mozart's sextet, on the contrary, is full of fast-paced action. In the preceding recitative the Count, Don Curzio, Marcellina, and Don Bartolo tried to coerce Figaro to marry Marcellina. The sextet begins when they discover that Marcellina is actually his mother, and Don Bartolo is his father. Susanna enters, and thinks that Figaro has betrayed her. In a comical commedia-del-arte routine, she asks each person in turn "His father? His mother?" Finally she is convinced, and the sextet ends with Susanna, Figaro, Mercellina, and Don Bartolo, now a happy family, presenting a united front against the Count and Don Curzio. The difference between this and Verdi's quartet is not merely the difference between tragedy and comedy. It is also the difference between a set piece elaborating a static situation, and one in which the situation is fluid, and develops as the music develops.

The problem of opera is, that the pace of events in music is slower than that in the theater. Music needs time to say what it has to say, with extended melodies, repetitions, variations, developments, recapitulations. Words and actions in the theater are more direct and less ambiguous, and therefore take less time.

In the operas of the seventeenth and eighteenth centuries, these two elements were not combined, but presented alternately. During the recitatives, the prose text was sung with speech rhythm, so that the play proceeded at a normal pace. This provided theater at the expense of music. During the set pieces -- arias, duets, ensembles, choruses, dances -- the drama paused until the music had run its course. This provided music at the expense of theater. From one point of view, such an opera is an overlong play in which the actors sing, made even longer by pieces of music inserted between the episodes of the play. From another point of view, it is an overlong concert, made even longer by sung conversations inserted between the pieces of music.

This unsatisfactory format, that stops and starts, became obsolete as part of the general upheaval at the time of the French Revolution. Mozart pointed the way to a new, more dynamic synthesis, but other composers did not (or were not able?) to follow his example. They made their operas less stylized by freeing themselves from traditional forms, but their ensembles, as set pieces, still stopped the drama dead in its tracks.

"Reform operas" did away with recitativo secco, and made the set pieces less tiresome. For an example, see: Gluck, Orfeo ed Euridice. Nevertheless, for an example of a static ensemble in a reform opera, see: Gluck, Iphigenie en Aulide act 3, quartet, Mon coeur saurait contenir [My heart can contain].

Some composers, perhaps taking their cue from Mozart's finales, employed chains of ensembles, in which each phase of the action is a new ensemble with its own tonality, meter, and thematic material. For an example, see Verdi, La Traviata finale of act 2 scene 2, Alfredo! Voi? [Alfredo! Is that you?] Nevertheless, for examples of static ensembles in operas with such finales, see:the Brindisi [drinking song] in La Traviata act 1, or the quartet in Rigoletto described above.

Some composers employed through-composed arioso held together by the motivic coherence of the music. For examples, see: Wagner's late operas, or Verdi, Falstaff. Nevertheless, for an example of a static ensemble in an opera with through-composed motivic arioso, see: Wagner, Die Meistersinger von Nurnberg act 2 scene 2, quintet, Selig wie die Sonne [Blissful as the sun].

Some composers employed complicated ensembles containing simultaneous but separate conversations. For an example, see: Puccini, La Boheme, act 2. Nevertheless, for an example of a static ensemble in an opera with such complex ensembles. see: Puccini, Tosca act 1, duet, Recondita armonia [Enigmatic harmony].

In spite of the subsequent innovations in opera, in spite of the freedom achieved by breaking away from traditional forms, Mozart's manner of composing ensembles is still unparalleled. None of the above, nor anyone else, has composed dynamic ensembles like those of Mozart. Detailed description of a few examples will demonstrate the originality and ingenuity of Mozart's incomparable ensembles.

Le Nozze di Figaro
act 1, duet, Cinque, dieci [Five, ten]
     Figaro and Susanna are discovered in a half-furnished room, each absorbed in his/her own activity. He is measuring the room by counting his steps; she is trying on a hat in front of a mirror.
     The orchestral introduction consists of two themes. The first, in m1-9, contains leaps of a fifth and a sixth; the second, in m9-18, contains flowing stepwise sequences.
     In m19-30 Figaro sings his measurements with the first theme: Cinque, dieci, venti, trenta [Five, ten, twenty, thirty]. In m30-36 Susanna sings her opinion of the hat with the second theme: Ora si, ch'io son contenta [Yes, now I am satisfied]. The two contrasting themes identify the two characters and their two separate preoccupations.
     In m36-49 they sing alternately, he continuing to measure, she appealing:to him: Guarda un po', mio caro Figaro [Look here a moment, my dear Figaro].
     The crucial moment occurs at m49, when he stops measuring and pays attention to the thing that interests her: Si, mio core [Yes, my heartthrob]. He sings this, not with his theme, but with her theme. From then on, his theme disappears, never to return, and they sing together with her theme.
     The way he puts aside his interest to share hers, establishes their affectionate relationship. In the following recitative she reciprocates, asking him what he was measuring -- and that sets the plot in motion.

Don Giovanni
act 1, quartet, Non ti fidar [Do not trust him]
     In the recitative that precedes this quartet, Donna Anna and Don Ottavio asked Don Giovanni to assist them in identifying the murderer of Donna Anna's father, unaware that he himself is the criminal. Don Giovanni mendaciously offered to help them with all the resources at his command.
     The quartet begins as Donna Elvira enters, and denounces Don Giovanni as a villain and a liar. Don Giovanni counters by saying that she is crazy. Donna Anna and Don Ottavio are irresolute bystanders.
     The crucial moment occurs at m66, when Donna Anna and Don Ottavio sing hesitantly: Incomincio a dubitar [I begin to doubt].
     Finally they are convinced. The quartet ends with Don Giovanni facing the united front of the other three.
     In the following recitative and aria, Donna Anna is sure that Don Giovanni is indeed the murderer, and she exhorts Don Ottavio to avenge the murder.

Le Nozze di Figaro
act 1, trio, Cosa sento? [What is this I hear?]
     At the beginning of the trio the Count is merely annoyed at Cherubino, but at the end he realizes that intrigues are afoot in the palace. Much of the thematic material of the music consists of sequences of one simple motive: three notes ascending or descending stepwise. The form of the trio is sonata-allegro form, no different from that of the first movements in Mozart's symphonies and sonatas (see my published article "Vocal Sonata Forms of Mozart"). The following description of the multi-layered complexity of this trio shows how the drama, the music, and the sonata-allegro form are congruent at every point.

Before the trio, Susanna had three unwelcome visitors who hid from each other. When the Count entered, Cherubino hid behind the armchair When Don Basilio entered, the Count hid behind the armchair, while Cherubino moved to the seat of the armchair and Susanna covered him with a robe.

m1-15 Cosa sento? Tosto andate, e scacciate il seduttor. [What is this I hear? Go at once, and expel the philanderer.]
     Drama The Count, angered by Don Basilio's gossip about Cherubino, jumps out of his hiding place. Music The motive ascending, once in the orchestra and then three times in both orchestra and voice. Form Beginning of the exposition; first theme.

m16-23 In mal punto son qui giunto. Perdonate,o mio signor. [My coming here is ill-timed. Pardon me, your lordship.]
     Drama Don Basilio answers apologetically and submissively. Music The motive descending four times. Inversion of the previous group. Form Continuation of the first theme.

m23-27 Che ruina! Me meschina! Sono oppressa dal terror. [What a calamity! How unfortunate for me! I am terrified.]
     Drama Susanna expresses her apprehension in an aside. Music Variant of the ascending motive, tremulous like her agitation. Form Continuation of the first theme.

m27-43 Each repeats his/her text.
     Drama Each continues as before. Music All sing simultaneously, Modulation to the dominant. Form Conclusion of the first theme.

m43-57 Ah, gia svien la poverina. [Ah, the poor thing has fainted.]
     Drama Susanna pretends to faint. The Count and Don Basilio take this opportunity to take hold of her. Music Begins with the ascending motive; ends with a descending sequence of descending skips of thirds, derived from the motive. Form Second theme, in the dominant.

m57-79 Pian, pianin [Slowly, carefully]
     Drama Susanna pretends to wake up. Form Continuation of the second theme.

m70-84 Siamo qui per aiutarvi. [We are here to help you.]
     Drama The Count and Don Basilio reassure Susanna. Music Repeat of the music of m43-57. Form Second theme, concluding the exposition.

m85-146 Ah, del paggio quel ch'ho detto [Ah, what I said about the page]
     Drama Conversation about Cherubino's mischief, during which the Count relates how he found him hiding in Barbarina's room. Music Free and rhetorical, even including several measures of recitative. Many references to the motive. Form Development section.

m129-146 The crucial moment occurs at the end of the development section: Ed alzando pian pianino [And slowly and carefully lifting the tablecloth].
     Drama The Count lifts the robe and discovers Cherubino hiding on the seat of the armchair. Music The motive descending, as the Count says that he lifted the tablecloth in Barbarina's room, then ascending as he suits the action to the words and lifts the robe. When Cherubino is revealed, the ascending motive is repeated three more times, as the Count exclaims: Ah, cosa veggio? [Ah, what is this I see?] -- and Susanna moans: Ah, crude stelle! [Ah cruel ill-starred disaster!] -- and Don Basilio gloats: Ah, meglio ancora! [Ah, better yet!]. Form End of the development section, leading to the beginning of the recapitulation.

m147-155 Onestissima siignora [Most honorable lady]
     Drama The Count now berates Susanna. Music First theme; same music for the Count, in the tonic, as in m6-11. Now interspersed with Susanna's asides. Form Beginning of the recapitulation.

m155-167 Cosi fan tutte [They all act like that]
     Drama Each continues as before. Music All sing simultaneously. Remains in the tonic. Form Continuation of the first theme.

m168-175 Each repeats his/her text.
     Drama Each continues as before. Music Second theme, but now in the tonic. Form Jumps directly to the second theme.

m176-182 Ah, del paggio quel ch'ho detto [Ah, what I said about the page]
     Drama Don Basilio repeats his m85-92, but now sarcastically. Music Continuation of the first theme. Form Recapitulation of Don Basilio's inversion of the first theme, but not in the same order as in the exposition.

m182-201 The trio concludes with all three singing simultaneously.
     Drama Susanna is still anxious: Accader non puo di peggio. Giusti Dei, che mai sara? [Things could not have turned out worse. Ye just gods. what will happen?] -- but Don Basilio is now cynical: Cosi fan tutte le belle. Non c'e alcuna novita. [All the pretty ladies act like that. It is nothing new.] -- and the Count now accuses Susanna of duplicity: Onestissima siignora, or capisco come va. [Most honorable lady, now I understand what is going on]. Music Second theme, now in the tonic. Form Conclusion of the recapitulation.

m201-221 Drama All three repeat the final words of their texts. Music The final cadence repeated several times. Form Coda.

In the recitative that follows the trio, the Count, realizing that something must be done, sends Cherubino to the army in order to get rid of him.

This last example emphasizes the unique nature of Mozart's ensembles. Only he could make a closed repetitive musical form congruent with the ongoing action of a play. Mozart's ensembles show how a musical form can be, not a straitjacket, but a drama. The recapitulation in a sonata-allegro is not merely a repetition, like the da-capo of a minuet. It is the same as the exposition, yet different because of what happened in the development. Therefore, the crucial moment in an operatic ensemble, when the situation changes, should occur at the seam between the development and the recapitulation.