Theoretical Grammar The subject-matter of theoretical grammar. Its relation to practical grammar

Qualitative vowel changes.
Development of monophthongs
The OE close labialized vowels [y] and [y:] disappeared in Early ME, merging with various sounds in different dialectal areas. The vowels [y] and [y:] existed in OE dialects up to the 10^th c., when they were replaced by [e], [e:] in Kentish and confused with [ie] and [ie:] or [i] and [i:] in WS. In Early ME the dialectal differences grew. In some areas OE [y], [y:] developed into [e], [e:], in others they changed to [i], [i:]; in the South-West and in the West Midlands the two vowels were for some time preserved as [y], [y:], but later were moved backward and merged with [u], [u:], e.g. OE fyllan – ME (Kentish) fellen, (West Midland and South Western) fullen, (East Midland and Northern) fillen – NE fill. In Early ME the long OE [a:] was narrowed to [o:]. This was and early instance of the growing tendency of all long monophthongs to become closer, so [a:] became [o:] in all the dialects except the Northern group, e.g. OE stān – ME (Northern) stan(e), (other dialects) stoon, stone – NE stone. The short OE [æ] was replaced in ME by the back vowel [a], e.g. OE þǽt > ME that [Өat] > NE that.
Development of diphthongs (see table)
OE possessed a well developed system of diphthongs: falling dphthongs with a closer nucleus and more open glide arranged in two symmetrical sets – long and short: [ea:], [eo:], [ie:] and [ea], [eo], [ie]. Towards the end of the OE period some of the diphthongs merged with monophthongs: all diphthongs were monophthongised before [xt], [x’t] and after [sk’]; the diphthongs [ie:], [ie] in Late WS fused with [y:], [y] or [i:], [i]. In Early ME the remaining diphthongs were also contracted to monophthongs: the long [ea:] coalesced (united) with the reflex of OE [ǽ:] – ME [ε:]; the short [ea] ceased to be distinguished from OE [æ] and became [a] in ME; the diphthongs [eo:], [eo] – as well as their dialectal variants [io:], [io] – fell together with the monophthongs [e:], [e], [i:], [i]. As a result of these changes the vowel system lost two sets of diphthongs, long and short. In the meantime anew set of diphthongs developed from some sequences of vowels and consonants due to the vocalization of OE [j] and [γ], that is to their change into vowels. In Early ME the sounds [j] and [γ] between and after vowels changed into [i] and [u] and formed diphthongs together with the preceding vowels, e.g. OE dæз > ME day [dai]. These changes gave rise to two sets of diphthongs: with i-glides and u-glides. The same types of diphthongs appeared also from other sources: the glide -u developed from OE [w] as in OE snāw, which became ME snow [snou], and before [x] and [l] as in Late ME smaul and taughte.
System of Vowels in Late ME (see table)