Vowels are represented by five letters and twenty phonemes divided into seven short vowels, for example /e/ and /ʌ/, five long vowels, for example /u:/ and /a :/, and eight diphthongs /ɪə/ and /ʊə/. Vowels are sounds produced by the complete passage of air through the vocal tract, without complete closure or narrowing. Unlike consonants, all vowel phonemes are voiced. When dividing words into syllables, each syllable must contain a vowel and this is called the syllable core. As with consonants, vowel phonemes are described based on the height and position of the tongue in the mouth and the shape of the lips. First, the height of the tongue in the mouth is divided into: high position as in the word keen /i:/, middle position as in the word belt /ɜː/, and low position as in the word cart /a:/. Second, the position of the tongue in the mouth is divided into: front position as in the word sheep /i:/, central position as in the word girl /ɜː/, and rear position as in the word pool /u:/. Third, lip positions are recognized as rounded as in the word horse /ɔː/ and not rounded as in the word ship.