In the globalized world we live in today international cooperation is extremely important, but not all Europeans can speak English very well. If the languages in question belong to the same family, each person might be able to speak their native language and understand the language of their interlocutor. Thus type of communication has proved to be effective between native speakers of Danish, Swedish and Norwegian; Dutch and German; as well as between the speakers of Czech and Slovak.
This thesis aims to discover how well speakers of different Slavic languages can understand each other and which factors influence this. About 12 000 speakers of six Slavic languages took part in a web-based intelligibility experiment. The findings show that the most important thing is how similarly words that have the same root are pronounced in two languages. Also, the more exposed you are to the language you are trying to understand, the better you will be at it. How words are formed and combined into sentences also matters, the more similar the processes, the more you are likely to understand. And in case that the level of intelligibility is relatively low, we showed that it is possible to teach people to understand a related language better in a relatively short time.