Wikipedia
Du-reformen (, the you-reform) was the process of popularization of the second-person singular pronoun du as a universal form of address in Sweden, in the late 1960s and early 1970s. The use of du ( cognate with English thou, French tu, and German du) replaced an intricate former system where speakers could use the third person hon and han ("she" and "he'), the second person plural pronoun ni ("you") or titles like fru and fröken (" Mrs." and " Ms."), kamrer ("accountant"), doktor ("doctor"), etc.
Formerly, it was considered impolite to address most people without an appropriate title. A subordinate was addressed as ni or han/hon, or alternatively by name. Du was only used between close friends, family, and for addressing children. However, usage of the singular and plural second-person pronoun or titles varied between different parts of the country and thereby also by social context, both before and after the reform.