Twelfth Night is filled with letters changing hands among characters, most notably Maria’s letter to Malvolio written in Olivia’s hand. There’s also the letter that Malvolio writes back to Olivia, as well as the letter that Andrew writes to Cesario ordering a duel (which Toby does not deliver). Beyond written letters, characters also deliver messages on behalf of others, as when Cesario tells Olivia of Orsino’s love and when Olivia sends Malvolio to deliver a ring to Cesario and tell them to return to see Olivia.

These messages are a way to facilitate communication between the characters and make their feelings known: Malvolio delivering the ring makes Viola realize that Olivia loves Cesario, for instance, while Malvolio’s letter back to Olivia allows him to assert that he’s not insane and believes Olivia has wronged him.

Furthering the play’s themes of deception and trickery, though, the letters and messages also facilitate miscommunication. Maria’s letter tricks Malvolio into believing Olivia is in love with him, and Sir Toby delivers false messages to Sir Andrew and Cesario, telling them that the other man is a valiant fighter determined to duel—even though both are actually cowardly and want to avoid a fight.