Although most dialects of Czech and Slovak are mutually intelligible (see Comparison of Slovak and Czech), eastern Slovak dialects are less intelligible to speakers of Czech and more so closer to Polish and mutual contact between speakers of Czech and speakers of the eastern dialects is limited.
Since the dissolution of Czechoslovakia it has been permitted to use Czech in TV broadcasting and—like any other language of the world—during court proceedings (Administration Procedure Act 99/1963 Zb.). From 1999 to August 2009, the Minority Language Act 184/1999 Z.z., in its section (§) 6, contained the variously interpreted unclear provision saying that "When applying this act, it holds that the use of the Czech language fulfills the requirement of fundamental intelligibility with the state language"; the state language is Slovak and the Minority Language Act basically refers to municipalities with more than 20% ethnic minority population (no such Czech municipalities are found in Slovakia). Since 1 September 2009 (due to an amendment to the State Language Act 270/1995 Z.z.) a language "fundamentally intelligible with the state language" (i.e. the Czech language) may be used in contact with state offices and bodies by its native speakers, and documents written in it and issued by bodies in the Czech Republic are officially accepted. Regardless of its official status, Czech is used commonly both in Slovak mass media and in daily communication by Czech natives as an equal language.
Czech and Slovak have a long history of interaction and mutual influence well before the creation of Czechoslovakia in 1918, a state which existed until 1993. Literary Slovak shares significant orthographic features with Czech, as well as technical and professional terminology dating from the Czechoslovak period, but phonetic, grammatical, and vocabulary differences do exist.
The Czech and Slovak languages form the Czech–Slovak (or Czecho–Slovak) subgroup within the West Slavic languages.
Most varieties of Czech and Slovak are mutually intelligible, forming a dialect continuum (spanning the intermediate Moravian dialects) rather than two clearly distinct languages; the eastern Slovak dialects are more divergent and form a dialect continuum with the Lechitic subgroup of West Slavic, most notably Polish.
The name Czechoslovak language is mostly reserved for an official written standard intended to unify Czech and Slovak created in the 19th century (but to a greater extent based on Czech rather than Slovak).