The changing use of “du” and “ni” in 20th century Swedish may be an interesting example of how rules of courtesy and formality may change.

(1) In earlier Swedish, one should use a title when addressing someone of higher social rank than oneself. Intimates could address each other with “du” (the regular 2nd person singular pronoun). A person of higher social rank could address someone of lower rank (for example, an employer talking to an employee) using “ni” (the 2nd person plural pronoun)

(2) In the early 20th century, there was attempts to change the formality system into French/German-style T/V-distinction, using “ni” as an all-purpose formal pronoun, while “du” would continue being the familiar pronoun. The attempts were only partly successful; many people (especially in more conservative small towns or rural areas) still considered “ni” as quite unfriendly, mainly used when addressing someone of a lower rank.

(3) In the latter half of the 20th century, in accordance with general social changes in a more egalitarian and familiar direction, “du” became a generally used pronoun, used on most social context (“du-reformen”, “the du-reform”).

(4) At the dusk of the 20th century, it became increasingly common for young people in various service occupations to address customers with “ni”, probably under influence from the French/German T/V-distinction. While it was done as an act of courtesy, the ironic result could be that old people, who had grown up with the earlier system, perceived it as rude and felt insulted.