The raising of long vowels and the lowering of short vowels, a phenomenon observed in a variety of languages (cf. Labov's (1996) Principles I and II), is to be explained as the speaker's capitalization on the compensatory perception of vowel height. Higher vowels sound longer than lower vowels, by way of compensation for the articulation-driven lengthening of open vowels. Speakers may exploit this effect by signalling short duration through vowel lowering and long duration by vowel raising. Two cases are presented in which the durational contrast in its turn serves to enhance a phonological opposition, a laryngeal opposition in English coda obstruents and a tonal opposition in Limburgian dialects of Dutch. A related correlation in vowel quality, the strengthening of diphthongal off-glides in short contexts versus the monophthongization in long contexts, is likewise to be explained as aiding the perception of the phonetic duration differences, although the effect is based on a different strategy, that of transferring the off-glide to a consonantal glide which is no longer included in the perceived vowel duration. Perceived vowel duration should be as carefully distinguished from acoustic vowel duration as pitch usually is from fundamental frequency. I will present the results of three experiments to support these claims.