Mongolian O: A Rounded Back Vowel Written Without Distinction

O writes a rounded back vowel and stands fourth in the alphabet. Its most important property is one it shares with the letter that follows it: the script writes o and u with a single shape and does not distinguish them. In practice a Mongolian word containing either is written with the same sign, and the reader supplies the difference from knowledge of the word. This is an inheritance rather than a decision.

The alphabet came to Mongolian from Old Uyghur, and behind that from Sogdian and ultimately from an Aramaic abjad in which a single letter covered the rounded vowels and the labial glide together. Mongolian took the sign as it stood and never subdivided it, so a system with more rounded vowels than its source is written with the source's inventory. Inside a word the shape is a rounded stroke attached to the spine; at the beginning of a word it is preceded by the opening stroke that introduces any vowel; at the end it takes a distinctive tail. Unicode encodes o at U+1823 and u at U+1824, so the two are separate characters in a digital text even though a handwritten page keeps them apart only in the reader's head. That gap between the encoded text and the written page is one of the practical difficulties of putting this script on a screen.