Theoretical linguistics is the branch of linguistics that is most concerned with developing models of linguistic knowledge. The fields that are generally considered the core of theoretical linguistics are syntax, phonology, morphology, and semantics. Although phonetics often guides phonology, it is often excluded from the purview of theoretical linguistics, along with psycholinguistics and sociolinguistics. Theoretical linguistics also involves the search for an explanation of linguistic universals, that is, properties all, or many, languages have in common.