This paper is fundamentally concerned with questioning the canonicity of the British Received Pronunciation (RP) system, which is almost normally taken to comprise forty-four (44) distinctive segmental sounds, i.e. phonemes. It proceeds by making use of the idea of ‘marginality’, defined as characterizing that which lies close to the margins or edges of any entity. A marginal phoneme is thus one which is at present excluded from the canon (but perhaps ought not to be), or alternatively one which is at present included in it (but shows signs of attrition). Sounds discussed in detail are triphthongs, certain diphthongs, the consonants /hw/ and /ʒ/, and the nasal vowel /ã/. At each point attention is drawn to the distinctive way in which each sound might be realized in Nigerian English. The main conclusion is that the triphthongs /aɪə/ and /aʊə/ clearly have phonemic status and ought to be included in the canon, and that the diphthongs /eə/ and /ʊə/ and the consonant /ʒ/ ought to be retained in it, so that RP would have a total of forty-six (46) phonemes. Some implications of the analysis for Nigerian English are briefly explored, and in particular the point is made that, given the reality of language change, it is not reasonable to expect that Nigerian English phonology should, for an indefinite future, reflect any changes that occur in British RP.