Many linguists believe that the language of our Indo-European ancestors had a considerable number of verbs which may appear both in intransitive and transitive constructions with no formal change in the verb, as in the case of English "The door opened" ~ "John opened the door" or Dutch "De sleutel draait in het slot" ("The key turns in the lock") ~ "Jan draait de sleutel in het slot" ("John turns the key in the lock"). Such verbs are called ‘labile’. However, and most amazingly, many other languages of the Indo-European family (such as Armenian, Hindi, Lithuanian or Russian) normally require different forms in different contexts. In other words, these languages have no or very few labile verbs. Such versatility observed within just one language family has puzzled linguists and Indo-Europeanists for more than 100 years. Although the phenomenon of lability is well-known from grammatical studies, its origin and evolution remains unclear. The research project focuses on the evolution of the system of labile verbs in several Indo-European language families (Indo-Aryan, Iranian, Greek, Italic, Romance, Germanic, Slavic).