Horse–hoarse merger

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

In rhotic dialects, /r/ is pronounced in most cases. In General American (GA), /r/ is pronounced as an approximant [ɹ] or [ɻ] in most positions, but after some vowels 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.

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