This cross-linguistic study investigates the functionality and use of one particular linguistic collocation in each of two languages, viz. the French c'est and the Swedish det ar, both meaning "it is." The data are drawn from conversational speech production by adult native speakers and second language learners. The investigation shows that these collocations are highly polyfunctional as components in different grammatical constructions, that they play a significant role as a formula in the planning and execution of utterances and that they show a characteristic distribution of frequency of use and a characteristic profile of development in learner speech. A usage-based view of how language develops in individuals is adopted in the analysis of these findings, showing that these various properties are connected to each other and that they have an important bearing on the development of learners' interlanguages. (Contains 8 tables. A list showing the Transcription conventions in the examples is appended.)