The intervocalic letters ɣ/g, and y has in some combinations come to help form long vowels, namely:[1]: 36–37
Long a with: aɣa, iɣa, iya.
Long e with: ege, ige, iye.
Long i with: igi.
Long o with: oɣa, oɣo, uɣa.
Long u with: aɣu, iɣu, uɣu.
Long ö with: öge, üge.
Long ü with: egü, igü, ügü.
Sometimes intervocalic b and m is silent: as in ᠳᠡᠪᠡᠯdebel (Khalkha: дээлdeel) 'robe, garment' or ᠬᠦᠮᠦᠨkümün (Khalkha: хүнkhün) 'human, person; man'.[2]: 64 [3]
The doubled vowels ii, uu, and üü mark these out as long. Doubled oo is instead both used in a few words to mark the vowel as short, and to distinguish it from u.[1]: 30
Most of the i's of these diphthongs derive from an earlier yi, but is no longer recognized as such. The yi origin can for instance be seen in the two long teeth of ᠰᠠᠶᠢᠨ⟨?⟩sayin 'good'. These has become a pair of short and long teeth (᠊ᠢ᠋) in recent manuscripts. The diphthongs only appears with the single form of i, as in ᠳᠠᠯᠠᠢdalai̯ 'sea', at the end of words.[1]: 10, 31 [4]: 9, 58
^Interrogative uu/üü particle (subject to vowel harmony; Khalkha: уу/үү/юу/юүuu/üü/yuu/yuü) used after the predicate.[7]: 437, 889, 1014 [1]: 172 [4]: 38 [8]: 53 [9]: 183 The positional variant ⟨ᠶᠤᠤ⟩yuu/yüü (Khalkha: юу/юүyuu/yuü) is only used in the modern language.[7]: 437 [8]: 53
^The tooth here is not from the semivowel y but is part of the first vowel itself, it's the only sign that distinguishes a u from a ü. Sample word: ᠦᠷ/ᠦᠦᠷür/üür (Khalkha: үүрüür) 'dawn, daybreak'.[7]: 1010, 1014
^Written with an intervocalic long tooth, as in the loanword ᠫᠦᠳ/ᠫᠦᠦᠳᠡpüd/püüde (Khalkha: пудpud) 'pud', a Russian weight.[7]: 650
^Final uu/üü in the prohibitive particle ᠪᠤᠤ()buu/büü (Khalkha: бүүbüü) 'don't' lacks an intervocalic long tooth.[7]: 141, 153 [1]: 166 [4]: 38 Contrast with the visually similar conjunction ᠪᠤᠶᠤ(): xiii buyu (Khalkha: буюуbuyuu) 'or',[7]: 132 [4]: 44 and noun ᠬᠦᠦküü (Khalkha: хүүkhüü) 'son, young boy', a colloquial form of ᠬᠥᠪᠡᠭᠦᠨköbegün (Khalkha: хөвүүн/хөвгүүнkhövüün/khövgüün),[7]: 494, 509 [1]: 20 [4]: 11 [10]: 816 [11]: 37 [12]: 395
^As in ᠠᠢai (Khalkha: айai) 'category; sound, noise', or an pity/sympathy/worry-expressing interjection.[7]: 19
^As in ᠡᠢei (Khalkha: ийii), an compassion/sorrow/fright/disgust-expressing interjection.[7]: 303
^As in ᠣᠢoi (Khalkha: ойoi) 'woods, forest, grove; mind, intellect; memory',[7]: 603–604 or ᠤᠢui (Khalkha: уйui) 'mourning, sorrow'.[7]: 866
^As in ᠥᠢ/ᠥᠶᠢöi/öyi (Khalkha: өөеööye), an exclamatory interjection meaning 'hello', 'I say', or 'look here',[7]: 633 [3] or ᠦᠢᠲᠦᠮᠡᠨüi (Khalkha: үйüi) tümen 'multitude; innumerable'.[7]: 999
^ abcdefghijklmnoLessing, Ferdinand (1960). Mongolian-English Dictionary(PDF). University of California Press. Note that this dictionary uses the transliterations c, ø, x, y, z, ai, and ei; instead of č, ö, q, ü, ǰ, ayi, and eyi;: xii as well as problematically and incorrectly treats all rounded vowels (o/u/ö/ü) after the initial syllable as u or ü.[6]