That is a good topic, Sazerac

, and as a French, I can tell you that choosing between "tu" and "vous" is not always as simple as it seems !
Basically, you would say "tu" to anyone in your family and any friend. You would say "vous" to someone you don't know or don't know well. You say "vous" in any professional context, except if it is the rule inside your company to say "tu" (which tends to be more and more the case). You say "vous" to people with a significant age difference with you, especially if they are quite older than you.
Think about "tu" as showing closeness and familiarity, while "vous" shows politeness and respect, but also distance.
One of the problems is going to "tu" when you have started to say "vous" to someone and are getting closer to him/her. I have the problem with my son's piano teacher ! I started to say "vous" as a sign of politeness, and now we both have trouble going to "tu" !
The way I was raised, I personnally have trouble saying "tu" to a new professional relation, even as I know it's the rule in the company I work in ...

But it seems that the younger people are, the more inclined they are to say "tu" to everyone. I remember having felt very cold when at 10 I went to middle school and all the teachers started to say "vous" to 10-years-old children ...
In some circumstances, saying "tu" can be pejorative, showing a lack of consideration. It happens, for example, that policemen say "tu" to colored people, showing by doing so a racist attitude. For saying "tu" to someone you don't know and in a strictly professional context is very rude indeed.
I have always wondered, you from the UK or US, what is your way to express that difference of attitude toward other persons, lacking the "tu" and "vous" distinction ?
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The world is my oyster !