Linguistics is the scientific study of language. Several of the subfields of linguistics that will be discussed here are concerned with the major components of language: Phonetics is concerned with the sounds of languages, phonology with the way sounds are used in individual languages, morphology with the structure of words, syntax with the structure of phrases and sentences, and semantics with the study of meaning. Another major subfield of linguistics, pragmatics, studies the interaction between language and the contexts in which it is used. Synchronic linguistics studies a language's form at a fixed time in history, past or present. Diachronic, or historical, linguistics, on the other hand, investigates the way a language changes over time. A number of linguistic fields study the relations between language and the subject matter of related academic disciplines, such as sociolinguistics (sociology and language) and psycholinguistics (psychology and language). In principle, applied linguistics is any application of linguistic methods or results to solve problems related to language, but in practice it tends to be restricted to second-language instruction.