Study of sound patterns within languages.
Diachronic (historical) phonology traces and analyzes changes in speech sounds and sound systems over time (e.g., the process by which sea and see , once pronounced with different vowel sounds, have come to be pronounced alike). Synchronic (descriptive) phonology investigates sound patterns at a single stage in a language's development, to identify which ones can occur and in what position (in English, for example, nt and rk appear within or at the end of words but not at the beginning).