Czech is spoken by roughly 10 million people, in at least 11 territories. After the Czech Republic, the majority of speakers are in Slovakia and other neighbouring territories. There are also speakers in the United Kingdom and the United States.
Slovak and Czech are mutually intelligible, and might be considered to be dialects of the same language. Their influence on each other predates the creation of Czechoslovakia.
Territory size shows the proportion of all people who speak Czech as a first language that live in that territory.