ending in , . , and , the final consonants are always preserved in so-called regular verbs, but in irregular verbs, and are lenited to and respectively while and are lost when followed by a vowel. Underlying consonant clusters While not permitted in the surface representation of Jeju, morpheme-final consonant clusters can exist in the underlying form. Many cases of post-sonorant aspiration involve morphemes whose Middle Korean cognates feature a final , suggesting that an underlying final after the sonorant should be posited in Jeju as well. Besides these -final clusters, Jeju permits a number of other final consonant clusters, including , , , , and (in the analysis of the government's orthography) . These clusters surface as a single consonant in isolation or before a consonant, but are fully realized when followed by a vowel. Vowels Jeju traditionally has a nine-vowel system: the eight vowels of Korean with the addition of ㆍ , a Middle Korean phoneme lost in Seoul in the eighteenth century. The phonemic identity of is controversial, but native speakers most commonly realize the phoneme as . and are only distinguished in the initial syllable. Among younger and less fluent speakers, and have both raised to and or respectively, resulting in a seven-vowel system identical to the vowel inventory of Seoul Korean. The raising of Jeju occurred before the raising of , and may have predated Standard Korean's ongoing merger of and . The subsequent loss of may have been motivated by a language-internal desire for symmetry in the vowel system. On the other hand, the vowel mergers are accelerated among Jeju speakers living in coastal communities more exposed to Standard Korean. Jeju has two or three glides: , , and possibly . can occur with all vowels except and . and have merged even among speakers who distinguish the monophthongs, and many speakers who retain also merge with . cannot occur with the three back vowels or with . occurs only with , and the resulting diphthong is generally realized as word-initially and otherwise. Glide-vowel sequences may be analyzed as diphthongs, with the phonemic identities of , , and being , , and respectively. Vowel phonological processes Several phonological processes affect the surface realization of Jeju vowels. In one process shared with Standard Korean, a bisyllabic vowel sequence may be contracted to a monosyllabic polyphthong. Vowel-affecting processes are particularly numerous in the verbal paradigm. Verb