Main Trends in the Changes of Stressed Vowels

§ 368. No other part of the English sound system has undergone such sweeping changes as the vowels in stressed syllables. They changed both in quality and quantity, under the influence of the environment and independently, alone and together with the surrounding sounds As a matter of fact, not a single OE long monophthong or diphthong has remained unaltered in the course of history; only a few short vowels were not changed, unless they were lengthened and then shared the fate of long vowels (for instance, short [i] and [o] have not suffered any changes in is and of — OE is, of, but the same sounds have developed into diphthongs if they became long: OE blind > ME blind [bli:nd] > NE blind, OE hopa > ME hope [hɔ:pə] > NE hope).

§ 389. The system of vowel phonemes has undergone drastic changes in the course of English linguistic history. Though the total number of phonemes has practically remained the same, their distinctive fea­tures and the principles of their opposition in the system, have altered.

Strictly speaking we can observe all kinds of vowel changes in all historical periods. And yet some prevailing trends of evolution can be singled out for certain groups of vowels at certain periods.

Long vowels were the most changeable and historically unstable group of English sounds. At all times they displayed a strong tendency to become narrower and to diphthongise, whereas short vowels displayed a reverse trend — towards greater openness, though this trend was less obvious and less consistent. Qualitative and quantitative changes were intertwined and often proceeded together.

It may be recalled here that in Early OE the prevalent type of vowel changes were assimilative changes mainly affecting the quality of the vowels. Towards the end of OE quantitative vowel changes gained mo­mentum. Early ME is mainly characterised by positional quantitative changes of monophthongs; at the same time profound independent changes affected the system of diphthongs: OE diphthongs were mono­phthongised and lost, and new types of diphthongs developed from vowels and consonants.

Late ME saw the beginnings of a new series of sweeping changes: independent qualitative changes of all long vowels known as the "Great Vowel Shift"; it lasted from the 14th till the 17th or even 18th c. Numer­ous positional vowel changes of this period — together with vocalisa­tion of consonants — gave rise to a number of new long monophthongs and diphthongs.