How closely related are the Czech and Slovak languages?

Czech and Slovak are closely related West Slavic languages that are mutually intelligible. The Western dialects of Slovak share similarities with the Moravian dialects of Czech. While both languages retain distinctions between long and short vowels and place stress on the first syllable, Slovak has historically been more phonetically conservative than Czech. This means Slovak has preserved certain sounds and diphthongs that have evolved or disappeared in Czech. Despite these differences, there are no sharp linguistic frontiers, and most dialects of both languages can be understood by speakers of the other.1234