Tybalt "sends a letter" to Romeo:

In lines 7-8, Benvolio tells us that Tybalt "hath sent a letter to his father's house." Shakespeare's audience apparently knew that such a letter was not a mere "Having a great time, wish you were here." When a person "sent a letter," that was a formal challenge to a duel. This was supposedly the equivalent of throwing down a gauntlet, or slapping Romeo in the face with a glove, or pouring rum in his jockey shorts... This, obviously, was a challenge to the death.

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