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How are Czech and Slovak related?

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Map of Europe showing the geographic distribution of Slavic languages, color-coded by language group: East Slavic (Russian, Ukrainian, Belarusian), West Slavic (Polish, Kashubian, Czech, Slovak, Sorbian), and South Slavic (Bosnian-Croatian-Montenegrin-Serbian, Slovene, Bulgarian, Macedonian). Striped areas indicate regions where bilingualism is common. — Slavic languages Distribution of the Slavic languages in Europe.

Czech and Slovak are closely related West Slavic languages that share a high degree of mutual intelligibility. Because there are no sharp linguistic frontiers between them, most dialects of the two languages are comprehensible to speakers of the other.12

Their relationship is characterized by both similarities and distinct evolutionary paths:

Linguistic Overlap: Western Slovak dialects share significant similarities with Moravian Czech dialects. Both languages utilize the Roman alphabet and retain distinctions between long and short vowels.1342
Divergence: Czech has undergone more extensive phonetic changes since the 14th century, whereas Slovak has remained more conservative, retaining long syllabic l and r sounds as well as specific diphthongs. Additionally, Czech developed a unique sibilant ř sound that is absent in Slovak.1
Historical Context: The two languages have a long history of interaction, including periods where Czech served as a literary and liturgical language in Slovakia. Following the formation of Czechoslovakia, fluency in both languages became a common legacy of their shared federation.52