2002, Cliff Goddard, Anna Wierzbicka (editors), Meaning and Universal Grammar:
In most Romance languages, the “intimate” T form is believed to be semantically more basic than the “formal” V form (Wierzbicka 1992:320).
2006, Anne Barron, Learning to say 'You' in German, in Language Learners in Study Abroad Contexts:
In general, the V form (Sie) is employed in German where interlocutors use a title and surname to refer nominally to each other, whereas the T form (du) is employed among interlocutors on first name terms (cf. Wein-rich, 1993: 822).
2010, Anna Trosborg (editor), Pragmatics across Languages and Cultures:
Historically, deciding whether to use the V or T form was determined by the power relationship between the interlocutors. Lower status persons addressed higher status persons with the V form; conversely higher status persons addressed lower status with the T form.
2015, Ronald Wardhaugh, Janet M. Fuller, An Introduction to Sociolinguistics:
The T form is sometimes described as the 'familiar' form and the V form as the 'polite' one, although the social meanings of these forms are in reality much more complex than that.