You mean, aside from Czechoslovakia? A long time ago they were one ethnic group, but "Historically, the region now known as Czech Republic was once part of the

You mean, aside from Czechoslovakia? A long time ago they were one ethnic group, but "Historically, the region now known as Czech Republic was once part of the Austrian empire whereas the Slovak area was a part of the Kingdom of Hungary" and so they kept different accents and slightly different vocabulary. They were separate for a long time, then tried to unify after WWI, but they've since split up. https://en.wikipedia.org/wiki/Czech%E2%80%93Slovak_languages https://en.wikipedia.org/wiki/Czech%E2%80%93Slovak_languages The languages of Thailand, Laos, and parts of southern China used to be the same about 1500 years ago.