The modern literary language developed during the 19th century. The most important event in its history was the 1850 "Vienna Agreement," a compromise between Croatian and Serbian literary figures and intellectuals that established the Štokavian dialect as the basis of a united literary language. The literary language has two major varieties: western, or Croatian, which is written in the Latin alphabet; and eastern, or Serbian, written primarily in the Cyrillic alphabet. A third standard is developing in Sarajevo and the rest of Bosnia. The differences between the varieties are almost entirely in vocabulary, with a few differences in syntax. The varieties have essentially identical sound systems. The literary language is notable for its vowel system. The vowels i, e, a, o, u, and r may be long or short (referring to the length of the vowel's sound) and may have rising or falling intonation. Thus the written word sela may have four different pronunciations and meanings, depending upon whether the first vowel is long or short and whether it has rising or falling intonation. This intonation system is unique among Slavic languages and can be met in Lithuanian or Latvian, the Baltic tongues.
Serbo-Croatian has preserved most of the cases from Common Slavic (the parent language of all the Slavic languages): nominative, genitive, dative, accusative, instrumental, and vocative. The locative case from Common Slavic has merged with the dative. Verb forms consist of a present tense, two future tenses, a past (perfect) tense, and a pluperfect tense. The literary language, especially in the eastern variant, has preserved two other past tenses from Common Slavic: imperfect and aorist.