The similarities between Czech and Slovak led to the languages being considered a single language by a group of 19th-century scholars who called themselves "Czechoslavs" (Čechoslováci), believing that the peoples were connected in a way which excluded German Bohemians and (to a lesser extent) Hungarians and other Slavs. During the First Czechoslovak Republic (1918–1938), although "Czechoslovak" was designated as the republic's official language both Czech and Slovak written standards were used. Standard written Slovak was partially modeled on literary Czech, and Czech was preferred for some official functions in the Slovak half of the republic. Czech influence on Slovak was protested by Slovak scholars, and when Slovakia broke off from Czechoslovakia in 1938 as the Slovak State (which then aligned with Nazi Germany in World War II) literary Slovak was deliberately distanced from Czech. When the Axis powers lost the war and Czechoslovakia reformed, Slovak developed somewhat on its own (with Czech influence); during the Prague Spring of 1968, Slovak gained independence from (and equality with) Czech. Since then, "Czechoslovak" refers to improvised pidgins of the languages which have arisen from the decrease in mutual intelligibility.

Who called themselves "Czechoslavs"?