English-language vowel changes before historic /r/

In English, many vowel shifts affect only vowels followed by /r/ in rhotic dialects, or vowels that were historically followed by /r/ that has been elided in non-rhotic dialects. Most of them involve the merging of vowel distinctions, so fewer vowel phonemes occur before /r/ than in other positions of a word.

In rhotic dialects, /r/ is pronounced in most cases. In General American English (GA), /r/ is pronounced as an approximant [ɹ] or [ɻ] in most positions, but after some vowels, it is pronounced as r-coloring. In Scottish English, /r/ is traditionally pronounced as a flap [ɾ] or trill [r], and there are no r-colored vowels.

In non-rhotic dialects like Received Pronunciation (RP), historic /r/ is elided at the end of a syllable, and if the preceding vowel is stressed, it undergoes compensatory lengthening or breaking (diphthongization). Thus, words that historically had /r/ often have long vowels or centering diphthongs ending in a schwa /ə/, or a diphthong followed by a schwa.

In most English dialects, there are vowel shifts that affect only vowels before /r/ or vowels that were historically followed by /r/. Vowel shifts before historical /r/ fall into two categories: mergers and splits. Mergers are more common, so most English dialects have fewer vowel distinctions before historical /r/ than in other positions of a word.

In many North American dialects, there are ten or eleven stressed monophthongs; only five or six vowel (rarely seven) contrasts are possible before a preconsonantal and word-final /r/ (beer, bear, burr, bar, bore, bor, boor). Often, more contrasts exist if /r/ appears between vowels of different syllables. In some American dialects and in most native English dialects outside North America, for example, mirror and nearer do not rhyme, and some or all of marry, merry, and Mary are pronounced distinctly. (In North America, those distinctions are most likely to occur in New York City, Philadelphia, some of Eastern New England (including Boston), and in conservative Southern accents.) In many dialects, however, the number of contrasts in that position tends to be reduced, and the tendency seems to be towards further reduction. The difference in how the reductions have been manifested represents one of the greatest sources of cross-dialect variation.

Non-rhotic accents in many cases show mergers in the same positions as rhotic accents even though there is often no /r/ phoneme present. That results partly from mergers that occurred before the /r/ was lost and partly from later mergers of the centering diphthongs and long vowels that resulted from the loss of /r/.

The phenomenon that occurs in many dialects of the United States is one of tense–lax neutralization in which the normal English distinction between tense and lax vowels is eliminated.

In some cases, the quality of a vowel before /r/ is different from the quality of the vowel elsewhere. For example, in some dialects of American English, the quality of the vowel in more typically does not occur except before /r/, and it is somewhere in between the vowels of maw and mow. It is similar to the vowel of the latter word but without the glide.