The Czech–Slovak group developed within West Slavic in the high medieval period, and the standardization of Czech and Slovak within the Czech–Slovak dialect continuum emerged in the early modern period. In the later 18th to mid-19th century, the modern written standard became codified in the context of the Czech National Revival. The main vernacular, known as Common Czech, is based on the vernacular of Prague, but is now spoken throughout most of the Czech Republic. The Moravian dialects spoken in the eastern part of the country are also classified as Czech, although some of their eastern variants are closer to Slovak.
Czech has a moderately-sized phoneme inventory, comprising ten monophthongs, three diphthongs and 25 consonants (divided into "hard", "neutral" and "soft" categories). Words may contain complicated consonant clusters or lack vowels altogether. Czech has a raised alveolar trill, which is not known to occur as a phoneme in any other language, represented by the grapheme ř. Czech uses a simple orthography which phonologists have used as a model.
Czech is a member of the West Slavic sub-branch of the Slavic branch of the Indo-European language family. This branch includes Polish, Kashubian, Upper and Lower Sorbian and Slovak. Slovak is the closest language genetic neighbor of Czech, followed by Polish and Silesian.
The West Slavic languages are spoken in Central Europe. Czech is distinguished from other West Slavic languages by a more-restricted distinction between "hard" and "soft" consonants (see Phonology below).
The term "Old Czech" is applied to the period predating the 16th century, with the earliest records of the high medieval period also classified as "early Old Czech", but the term "Medieval Czech" is also used.