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Serbo-Croatian was standardized in the mid-19th-century Vienna Literary Agreement by Croatian and Serbian writers and philologists, decades before a Yugoslav state was established. From the very beginning, there were slightly different literary Serbian and Croatian standards, although both were based on the same Shtokavian subdialect, Eastern Herzegovinian. In the 20th century, Serbo-Croatian served as the official language of the Kingdom of Yugoslavia (when it was called "Serbo-Croato-Slovenian"), and later as one of the official languages of the Socialist Federal Republic of Yugoslavia. The breakup of Yugoslavia affected language attitudes, so that social conceptions of the language separated on ethnic and political lines. Since the breakup of Yugoslavia, Bosnian has likewise been established as an official standard in Bosnia and Herzegovina, and there is an ongoing movement to codify a separate Montenegrin standard. Serbo-Croatian thus generally goes by the ethnic names Serbian, Croatian, Bosnian, and sometimes Montenegrin and Bunjevac.
What was Serbo-Croatian the official language for in the 20th century?
Kingdom of Yugoslavia