The ancestors of the Slovaks and Moravians were later united in Great Moravia between 833 and 907. Its core territory is the region now called Moravia in the eastern part of the Czech Republic alongside the Morava River, which gave its name to the kingdom. The kingdom saw the rise of the first ever Slavic literary culture in the Old Church Slavonic language as well as the expansion of Christianity after the arrival of St. Cyril and St. Methodius in 863 and the creation of the Glagolitic alphabet, the first alphabet dedicated to a Slavonic language, which had significant impact on most Slavic languages and stood at the beginning of the modern Cyrillic alphabet.
The Czechs were only part of Great Moravia for some seven years before splitting from it in 895. However, already at the age of Great Moravia, we can see the basis for their national identity. The former Coat of Arms of the region of Moravia from that age is still present in current Coat of Arms of the Czech Republic.
Furthermore, in the second half of the 10th century, the Czechs conquered and controlled western Slovakia for around thirty years. This was the last time the two nations were united; the Hungarians had conquered Slovakia by the 11th century, while the Czechs maintained their own principality (a kingdom since 1198) of Bohemia, from around 900 to 1919.
Both Czechs and Slovaks struggled against a powerful neighbouring people; Germans in the case of the Czechs, Hungarians in the case of the Slovaks (see History of the Czech Republic and History of Slovakia). Contacts between the Czechs and Slovaks arose in:
- the late 14th century, when Slovaks started to study at the University of Prague
- in the 15th century, with the campaigns of the Czech Hussite armies to Slovakia
- in the 17th century, when Czech Protestants fled to Slovakia
Between the 15th and 18th centuries, some educated Slovaks used written Czech as well as Slovak and Latin. The Czechs and Slovaks were also formally united in 1436–1439, 1453–1457, and 1490–1918, when Hungary (which included Slovakia), Bohemia and other Central European states were ruled by the same kings.