Historical linguistics
Historical linguistics is the study of how languages change over time. Whereas etymology is concerned with the origin of words, historical lingistics forms a range of inquiries that attempt to identify patterns in language change - for example, how rules of grammatical structure (syntax) emerge and vary over time.
Prior to the rise of modern theoretical linguistics, the precursors of today's historical linguistics were the dominant fields of linguistic study. Linguists often studied related languages to discover the language they were thought to originate from (comparative philology), and tried to identify predictable sound changes - for example, the well-attested change of p to f was identified as Grimm's law through study of Indo-European languages. Today, the study of related tongues goes on as comparative linguistics.