This paper proposes a functional basis for final consonant extrametricality, the asymmetric status of CVC syllables as stress-attracting in non-final position of a word but stress-rejecting in final position. A typological study of phonemic vowel length pattern in 10 languages with this final vs. non-final stress asymmetry and 30 languages in which CVC attracts stress in final position indicates a robust asymmetry between languages differing in their stress system’s treatment of final CVC. Languages that asymmetrically allow stress on non-final but not on final CVC all lack phonemic vowel length contrast in final position, whereas those lacking the stress asymmetry often have contrastive length in final vowels. It is claimed that the absence of phonemic length in languages that do not stress final CVC facilitates the nearly universal pattern of phonetic final lengthening, which threatens to obscure the perception of phonemic length. The enhanced lengthening of final vowels in languages with final phonemic vowel length reduces the duration ratio of CVC relative to CV, thereby reducing CVC’s perceptual prominence and thus its propensity to attract stress in keeping with Lunden’s (2006) proportional duration theory of weight. A phonetic study of two languages differing in the stress-attracting ability of final CVC offers support for the proposed account. Arabic, which displays consonant extrametricality and largely lacks phonemic vowel length in final position, has substantial final vowel lengthening, whereas Kabardian, which stresses final CVC and contrasts vowel length in final position, lacks substantial final lengthening.