Vowels before /r/ in the history of English
In the past few centuries vowels before historic /r/ have gone through many changes in different varieties of English, including non-rhotic forms which lost syllable-final /r/ in the eighteenth or early nineteenth centuries. These changes can be grouped into two major types. The first is characterized by the collapse of a front/back distinction for short-mid vowels (the NURSE-TERM merger) which holds for all supraregional forms of English, bar those in Scotland where, in addition, the vowel in BIRD may retain a front, high quality. The loss of distinctiveness for these vowels is attributed to the rhotacisation of the entire syllable nucleus which is something which must have happened before the loss of /r/ which led to non-rhotic varieties. The second type of change involves the merger of two formerly distinct pre-rhotic vowels to one, as in the HORSE-HOARSE and the POOR-POUR mergers. There are further subtypes to the merger development, e.g. merger through diphthong smoothing as in the TOWER-TYRE merger. Finally, there are pre-rhotic mergers which involve more than two elements, e.g. the MARY-MERRY-MARRY which depend on the loss of both qualitative and quantitative distinctions.