Serbo-Croatian () – also called Serbo-Croat (), Serbo-Croat-Bosnian (SCB), Bosnian-Croatian-Serbian (BCS), and Bosnian-Croatian-Montenegrin-Serbian (BCMS) – is a South Slavic language and the primary language of Serbia, Croatia, Bosnia and Herzegovina, and Montenegro. It is a pluricentric language with four mutually intelligible standard varieties, namely Serbian, Croatian, Bosnian, and Montenegrin. South Slavic languages historically formed a continuum. The turbulent history of the area, particularly due to expansion of the Ottoman Empire, resulted in a patchwork of dialectal and religious differences. Due to population migrations, Shtokavian became the most widespread dialect in the western Balkans, intruding westwards into the area previously occupied by Chakavian and Kajkavian (which further blend into Slovenian in the northwest). Bosniaks, Croats and Serbs differ in religion and were historically often part of different cultural circles, although a large part of the nations have lived side by side under foreign overlords. During that period, the language was referred to under a variety of names, such as "Slavic" in general or "Serbian", "Croatian" or "Bosnian" in particular. In a classicizing manner, it was also referred to as "Illyrian". The process of linguistic standardization of Serbo-Croatian was originally initiated 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 dialect of Shtokavian, 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 along 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. Like other South Slavic languages, Serbo-Croatian has a simple phonology, with the common five-vowel system and twenty-five consonants. Its grammar evolved from Common Slavic, with complex inflection, preserving seven grammatical cases in nouns, pronouns, and adjectives. Verbs exhibit imperfective or perfective aspect, with a moderately complex tense system. Serbo-Croatian is a pro-drop language with flexible word order, subject–verb–object being the default. It can be written in either localized variants of Latin (Gaj's Latin alphabet, Montenegrin Latin) or Cyrillic (Serbian Cyrillic, Montenegrin