The study of phonology as it exists today is defined by the formative studies of the 19th-century Polish scholar Jan Baudouin de Courtenay, who (together with his students Mikołaj Kruszewski and Lev Shcherba in the Kazan School) shaped the modern usage of the term phoneme in a series of lectures in 1876–1877.
Historical phonology focuses on the nature of linguistic change itself, that is, how and why languages change, and the underlying forces and processes which shape, mould and direct modifications.
Old English had a moderately large vowel system.
In stressed syllables, both monophthongs and diphthongs had short and long versions, which were clearly distinguished in pronunciation.
In unstressed syllables, vowels were reduced or elided, though not as much as in Modern English.