Mongolian U: The Rounded Vowel Written as O

U writes a rounded back vowel and is the fifth letter of the alphabet. It shares its written shape completely with the letter before it, so that in ordinary writing there is no difference at all between o and u, and the two are kept apart only in the reader's knowledge of the word. The alphabet nevertheless lists them as separate letters, recites them separately, and encodes them separately at U+1823 and U+1824, which tells you that the tradition regards them as two letters that happen to look alike rather than as one letter with two readings. The distinction is real in the language even where it is invisible on the page. Both belong to the back class of the vowel harmony, so neither helps a reader decide which class a word belongs to; that job falls to the consonants and to the suffixes.

In the sequence of positional forms, u behaves exactly as o does, with an initial form introduced by an opening stroke, a medial form attached to the spine, and a final form drawn out below the last letter of the word. Learners of the script are usually taught the o and u pair together and taught immediately that the pair cannot be resolved from the shape, which is a useful early lesson in how much of this writing system depends on the reader.