The usage of Czech in a Slovak environment resulted in the Slovakized Czech language, a variant of cultural Czech with Slovak elements. This variant existed from the penetration of Czech to present-day Slovakia and was used in city books and official correspondence. Early writings had a varying frequency of Slovak elements caused by a poor knowledge of standard Czech among many Slovak native speakers and the influence of vernacular language and cultural Slovak. The normalized form of Slovakized Czech existed from the 17th century. In it, Czech letters and words were systematically replaced by their Slovak equivalents (e.g. "ř" by "r", "ě" by "e", "au" by "ú", "ou" by "ú", etc.).