There was no standardization distinguishing between Czech and Slovak prior to the 15th century. In the 16th century, the division between Czech and Slovak becomes apparent, marking the confessional division between Lutheran Protestants in Slovakia using Czech orthography and Catholics, especially Slovak Jesuits, beginning to use a separate Slovak orthography based on the language of the Trnava region.
The publication of the Kralice Bible between 1579 and 1593 (the first complete Czech translation of the Bible from the original languages) became very important for standardization of the Czech language in the following centuries.
In 1615, the Bohemian diet tried to declare Czech to be the only official language of the kingdom. After the Bohemian Revolt (of predominantly Protestant aristocracy) which was defeated by the Habsburgs in 1620, the Protestant intellectuals had to leave the country. This emigration together with other consequences of the Thirty Years' War had a negative impact on the further use of the Czech language. In 1627, Czech and German became official languages of the Kingdom of Bohemia and in the 18th century German became dominant in Bohemia and Moravia, especially among the upper classes.
Czech contains ten basic vowel phonemes, and three more found only in loanwords. They are /a/, //, //, /o/, and /u/, their long counterparts /a/, //, /i/, /o/ and /u/, and three diphthongs, /ou/, /au/ and /u/. The latter two diphthongs and the long /o/ are exclusive to loanwords. Vowels are never reduced to schwa sounds when unstressed. Each word usually has primary stress on its first syllable, except for enclitics (minor, monosyllabic, unstressed syllables). In all words of more than two syllables, every odd-numbered syllable receives secondary stress. Stress is unrelated to vowel length, and the possibility of stressed short vowels and unstressed long vowels can be confusing to students whose native language combines the features (such as English).
Voiced consonants with unvoiced counterparts are unvoiced at the end of a word, or when they are followed by unvoiced consonants. Czech consonants are categorized as "hard", "neutral" or "soft":
Hard: /d/, //, //, /k/, /n/, /r/, /t/, /x/
Neutral: /b/, /f/, /l/, /m/, /p/, /s/, /v/, /z/
Soft: /c/, //, /j/, //, /r/, //, /ts/, /t/, //
This distinction describes the declension patterns of nouns, which is based on the category of a noun's ending consonant. Hard consonants may not be followed by i or in writing, or soft ones by y or (except in loanwords such as kilogram). Neutral consonants may take either character. Hard consonants are sometimes known as "strong", and soft ones as "weak".
The phoneme represented by the letter (capital ) is considered unique to Czech. It represents the raised alveolar non-sonorant trill (IPA: [r]), a sound somewhere between Czech's r and (example: "eka" (river) ), and is present in Dvok.
The consonants /r/ and /l/ can be syllabic, acting as syllable nuclei in place of a vowel. This can be difficult for non-native speakers to pronounce, and Str prst skrz krk ("Stick [your] finger down [your] throat") is a Czech tongue twister.