Most varieties of Czech and Slovak are mutually intelligible, forming a dialect continuum (spanning the intermediate Moravian dialects) rather than being two clearly distinct languages; standardised forms of these two languages are, however, easily distinguishable and recognizable because of disparate vocabulary, orthography, pronunciation, phonology, suffixes and prefixes.

The name "Czechoslovak language" is mostly reserved for an official written standard devised in the 19th century that was intended to unify Czech and Slovak.

It was proclaimed an official language of Czechoslovakia and functioned de facto as Czech with slight Slovak input.

Prior to the Magyar invasion of Pannonia in the 890s, the West Slavic polity of Great Moravia spanned much of Central Europe between what is now Eastern Germany and Western Romania.

In the high medieval period, the West Slavic tribes were again pushed to the east by the incipient German Ostsiedlung, decisively so following the Wendish Crusade in the 11th century.

For example, the spirantisation of Slavic /g/ to /h/ is an areal feature shared by the Czech-Slovak group with both Ukrainian and Sorbian (but not with Polish).

The Slovaks, on the other hand, never became part of the Holy Roman Empire in the medieval period, being incorporated into the Kingdom of Hungary.

[7] The Czech-Slovak dialect continuum historically blended into Silesian in the west and Old Ruthenian (also known as Chancery Slavonic) in the east.

[8] In a 1964 textbook on Czech dialectology, Břetislav Koudela used the sentence put the flour from the mill in the cart to highlight phonetic differences between dialects:[12]