The paper analyses the position of Czech language as one of the two elements of diglossia in Slovak culture and literature between 1780 and 1843, when Ľudovít Štúr’s codification was adopted. It discusses the relationship between Czech and the first norm of standard Slovak, codified by Anton Bernolák in 1787. The focus is on linguistic tendencies that brought Czech closer to pre-standard forms of Slovak, based on Central Slovak dialects. An analysis is undertaken of orthographic, grammatical and lexical modifications of the Czech orthographic norm in the territory of today’s Slovakia, as well as their forms in literary texts by Slovak writers. The paper also introduces Slovak theoretical works that concentrate on researching adaptations of the Czech orthographic norm in the Slovak milieu. Linguistic and literary analyses of the situation in Slovak literature in the first half of the nineteenth century reveal a significant weakening of the position of Czech in Slovakia. This is confirmed by the strong Slovakisation of Czech in the 1830s and 1840s, as well as Štúr’s codification of standard Slovak in 1843, which concluded the process of the language integration of Slovaks.