Linguistics is the study of these knowledge systems in all their aspects: how such a knowledge system is structured, how it is acquired, how it is used in the production and comprehension of messages, how it changes over time. Linguistics can be defined as the scientific study of human language. Linguists focus on describing and explaining language. The scope of linguistics includes both language structure and language use. The part linguistics that is concerned with the structure of language is divided into a number of subfields: