The 1st problem of the phonological analysis is to establish phonemes in a language. There are some difficulties in English:
1. IF THERE IS A SCHWA-VOWEL PHONEME? Though the schwa-vowel can be opposed only to weakened vowel phonemes, which are partially reduced due to their position in unstressed syllables, it can form phonological oppositions with the number of other phonemes and can distinguish words (accept-except, solar-solo).
2. IF DIPHTHONGS AND TRIPHTHONGS ARE MONOPHONEMIC OR BIPHONEMIC CLUSTERS? Trubetzkoy worked out a number of rules which help to determine whether a sound of a complex nature is monophonemic: 1) a phoneme is indivisible, as no syllable division can occur within a phoneme; 2) a phoneme is produced by 1 articulatory effort; 3) the duration of a phoneme should not exceed that of other phonemes in the language => 1. The syllabis and articulatory indivisibility of diphthongs and their duration of English historically vowels, clearly determine their monophonemic character. 2. triphthongs [aue, aie] are not produced by a single articulatory effort, as there is an increase in the force of articulation and intensity for the 1st and the last element. The syllabic division generally occurs in between the diphthong and the schwa-vowel. So they are regarded as biphonemic clusters.
In such a way it has been established that in RP there are 20 vowel phonemes (12 monophthongs, 8 diphthongs).