http://linguaphiles.livejournal.com/5976046.html

I've been involved in a discussion about the use of the familiar you (in languages that have a familiar and formal version). In English, that used to be "thou" and "thee," in French, Spanish, etc., "tu," and in German "du." Of course in everyday use the familiar is for those close to you or younger, while the formal "vous" etc. is for strangers, elders, and others you must address with respect. However, the familiar in modern English is mainly known to most of us from the Lord's Prayer, and I see that in many other languages the familiar form is also used in that prayer.

I thought I remembered that in French, the familiar "tu" is used to address God and would be used to address royalty, if there still were French royalty. Does anyone know how this special use of the familiar came about? Is it true in other languages besides French, and was it true in Latin?