succession, or as a monosyllabic triphthong. In more casual speech the middle vowel may be considerably reduced, by a process known as smoothing, and in an extreme form of this process the triphthong may even be reduced to a single long vowel. In such a case the difference between , , and in tower, tire, and tar may be neutralised with all three units realised as or . This type of smoothing is known as the tower–tire, tower–tar and tire–tar mergers. BATH vowel There are differing opinions as to whether in the BATH lexical set can be considered RP. The pronunciations with are invariably accepted as RP. The English Pronouncing Dictionary does not admit in BATH words and the Longman Pronunciation Dictionary lists them with a § marker of non-RP status. John Wells wrote in a blog entry on 16 March 2012 that when growing up in the north of England he used in "bath" and "glass", and considers this the only acceptable phoneme in RP. Others have argued that is too categorical in the north of England to be excluded. Clive Upton believes that in these words must be considered within RP and has called the opposing view "south-centric". Upton's Oxford Dictionary of Pronunciation for Current English gives both variants for BATH words. A. F. Gupta's survey of mostly middle-class students found that was used by almost everyone who was from clearly north of the isogloss for BATH words. She wrote, "There is no justification for the claims by Wells and Mugglestone that this is a sociolinguistic variable in the north, though it is a sociolinguistic variable on the areas on the border [the isogloss between north and south]". In a study of speech in West Yorkshire, K. M. Petyt wrote that "the amount of usage is too low to correlate meaningfully with the usual factors", having found only two speakers (both having attended boarding schools in the south) who consistently used . Jack Windsor Lewis has noted that the Oxford Dictionary's position has changed several times on whether to include short within its prescribed pronunciation. The BBC Pronouncing Dictionary of British Names uses only , but its author, Graham Pointon, has stated on his blog that he finds both variants to be acceptable in place names. Some research has concluded that many people in the North of England have a dislike of the vowel in BATH words. A.