Cos fan tutte is a two-act opera buffa by Wolfgang Amadeus Mozart, an Austrian composer best known for his symphonies and operas such as Don Giovanni, The Magic Flute, and The Marriage of Figaro. Cos fan tutte was written in 1789 with an original libretto by Lorenzo da Ponte, who also wrote the librettos for The Marriage of Figaro and Don Giovanni. It was premiered at the Burgtheater in Vienna on January 26, 1790.
Cos fan tutte is set in eighteenth-century Naples and takes place over the course of a single day. Ferrando and Guglielmo, two officers, fall in love with two sisters, Dorabella and Fiordiligi, respectively. Hearing the men praise the women, an old cynic, Don Alfonso, tells them that there is no such thing as a faithful woman, and when they argue, he proposes a bet. He bets that if they leave for a single day, the sisters will prove unfaithful, which he plans to demonstrate by having them pretend to leave, return in disguise, and woo the sisters.
The sisters are upset by the men’s departure and, after bidding them farewell, explain the situation to their maid, Despina, whom Alfonso quickly drags into the wager. The two soldiers enter disguised as poisoned soldiers and are left in the care of the sisters, while Despina and Alfonso seek assistance, returning with Despina disguised as a doctor who pretends to cure the men. The sisters both refuse the revived men’s request for a kiss.
The men continue the siege in Act II of Cos fan tutte, with the disguised Ferrando pursuing Fiordiligi and Guglielmo making progress with Dorabella. Soon, the men have achieved such success that a lawyer, this time Despina in disguise, is summoned to draft marriage contracts. Just before the contracts are signed, Don Alfonso makes a well-timed entrance to announce the soldiers’ return. Don Alfonso hands them the marriage contracts as proof of their betrayal as the disguised men flee and reenter as themselves. Until Don Alfonso reveals the wager, everything appears to be at a standstill.
The couples are paired off at the end of Cos fan tutte, but neither the libretto nor the music indicate whether they are the original couples or the ones formed when the men were in disguise. This leaves it up to the director to decide how each performance of the opera should end.