This paper demonstrates that the vernacular of Czech Jewry inhabiting Prague during the Přemyslid reign (11th-14th century), was in essence similar to modern Prague Czech. The corpus of Canaanite, i.e., Slavic, in particular Old Czech glosses in Hebrew script is exceptional for its orthography, phonology, lexicology and grammar. The paper debates the linguistic situation regarding multilingualism, Slavic explanation of the glosses, their functions and properties and various sociolinguistic aspects like archaisation, name giving and place names in communication.