original_query: What is the relationship between the pronunciation of "r" and voiceless fricatives in certain dialects?
query: What is the relationship between the pronunciation of "r" and voiceless fricatives in certain dialects, specifically concerning vowel lengthening and raising?
answer: In some dialects, the presence of a voiceless fricative following a vowel (like /r/ in 'car') triggers lengthening and raising of the preceding vowel.  This is particularly relevant in the evolution of the 'lot-cloth' split and is most consistent in the final syllable of a word.