Du-reformen (Swedish pronunciation: [ˈdʉ̂ːrɛˌfɔrmɛn], "the thou-reform") was the process of popularization of the second-person singular pronoun du as a universal form of address in Sweden that took place in the late 1960s. The use of du (cognate with English thou, French tu, and German du) replaced an intricate former system where people chiefly addressed each other in third person, with or without a preceding Mr./Mrs./Ms. (herr, fru or fröken) before the title, often omitting both surname and given name. Less respectfully, people could be addressed with Mr./Mrs./Ms. plus surname, or in a family setting, alternatively even less respectfully, with the plain name or third person pronoun han ('he') or hon ('she').
Before du-reformen, it was considered impolite to address most people without an appropriate title, although a subordinate could be addressed by name, or less respectfully as Ni or han/hon. The informal du had been used for addressing children, siblings, close friends and possibly for cousins, but hardly for elder relatives. With exceptions in dialectal usage, the second-person plural pronoun Ni had long been considered degrading when used to address a single person. However, the usage varied between different parts of the country, and as well by social context, both before and after the reform.
Finland Swedish has undergone a similar development, but slower and slightly less due to influence of the somewhat more conservative usage in Finnish. In particular, the use of second-person plural (Ni) to address an unknown person is considered respectful or neutral rather than degrading.