Length

Length of vowels (and diphthongs) varies predictably according to the context they appear in. More specifically:

(a) Vowels are longer before voiced consonants than before their voiceless counterparts. Thus, the phonetic realization of the vowel /æ/ in bag [bæg] is longer than its realization in back [bæk].

(b) Vowels are longer before sonorant consonants than before obstruents. Thus the phonetic realization of the vowel /o/ in goal [gol] is longer than its realization in goad [god].

Vowels are longer in open syllables than in closed syllables. Thus, the phonetic realization of the vowel /e/ in bay [be] is longer than its realization in bait [bet].

We can combine the three rules above and say that we find a vowel longest in an open syllable (e.g. /i/ in knee [ni]); next longest in a syllable closed by a sonorant consonant (e.g. /i/ in kneel [nil]); next longest in a syllable closed by a voiced obstruent (e.g. /i/ in need [nid]); and shortest in a syllable closed by a voiceless consonant (e.g. /i/ in neat [nit]).

(d) Vowels are longer in stressed syllables than in unstressed syllables. Thus, the phonetic realization of the phoneme /i/ in the stressed (bold-faced) syllable of the word appreciate [ə.pɹ̣i.ʃi.et] is longer than its realization in the following unstressed syllable.