In French, tu is the singular familiar "you" and vous is the plural "you" or the singular formal "you". This is a problem for English-speaking people who learn French, but also for French translators when the original English author said "you". Usage of vous as the plural "you" is easy, so I will describe when you should use tu (tutoiement) or vous (vouvoiement) as a singular in French. I'll try to be as complete as possible.

You will say tu to someone when you:

  • have a personal relationship with that person. You will say tu to relatives and friends. You may start saying vous to a girl (or a boy) and turn to tu after you kiss or have sex with her/him (but you may also say tu from the beginning if you have been introduced by other friends).
  • are equal on some respect. You may say tu to your colleagues (but that depends on how much relaxed the atmosphere is in your company) or even to people who graduated from the same school as you did. For example, former students of Ecole Polytechnique are known for saying tu to one another, even if one of them graduated thirty years before the other.
  • are a superior. You will say tu to a child until 15 or 18. If you want to please a teenager whom you don't know personally, say vous to him and he will feel important.
  • despise the person you are talking to. When you are very angry, you may say tu to someone you don't know. A car driver will typically say to "Va te faire foutre" (fuck you) to another car driver, rarely "Allez vous faire foutre" (1).
    The opposite happens too. An angry husband may suddenly say vous and Madame to his wife.
  • are chatting on the Internet. However some people prefer vous even on the Internet, so the best thing is to say vous to these people. When in doubt, say vous when the topic of the chat room or newsgroup is professional and tu when it's fun (which would include Everything2 if the language was French here).
  • are a very young child (under 6 or 8). Some very young children say tu to everybody. But not all of them. When I was 6, my school teacher was also my mother, and I had to say vous to her in the classroom because all the other kids did. Two years later, she moved to another school where the children said tu to their teacher. At first she was surprised, then she got used to it.
  • are speaking to God. She will answer you the same way.
  • are communist speaking to a comrad.
  • are very cool, dude, and are familiar with everybody you see in the street. A classic humorist joke consists in saying tu to famous politicians and watching their reaction.

In practice, if you are a foreigner, the following rules should be sufficient:

Deciding between tu and vous is not always easy, even for French people. When you have known someone for months, you feel it's time to start calling him tu. But how to do it, and how to change long-time habits? Who will start first? My father and his neighbors have been saying vous for 20 years although they were good friends. Then a new family came to live there in the street; they were nice people and they started to say tu to us. Now everybody says tu in the neighborhood.

Usage of singular vous conflicts with grammar rules about plural. If we want to say "you have come" to the President, should we say "vous êtes venu" (plural adjective) or "vous êtes venus" (singular adjective)? The right answer is the first one: "vous êtes venu". It may or may not seem logical to you: since we are using the plural for "you" to show respect, why not be consistent and use the plural for the adjective too?

Vouvoiement probably dates from the Roman Empire. According to Grévisse's "Le bon usage", Ovid used it, and Gritchka reports a similar usage of vos by Catullus (see tu). Much later,in year 293, Diocletian divided the Empire in four territories, and placed each area under the authority of a tetrarch. Because each tetrarch represented the authority of the Empire, it became usual to say vos to them instead of tu. The system collapsed when Diocletian resigned. Vouvoiement may also be related to nounoiement (see je versus nous).

xmatt: yes, nowadays most people say tu to everybody in their family, even distant cousins. But it was not true 100 years ago, and it is still not true in some upper-class families. For example, I believe President Jacques Chirac says vous to his wife.

(1) One day a 13-year old student said "Ta gueule" (shut up, very rude) to one of my teachers. The teacher, who was a very polite woman, said: "You (vous) should be more polite with me". The student repeated: "Votre gueule" (with a formal "you").