ISO 10646/Unicode 3.0 defines Korean Hangul Jamos (alphabetic letters) at [1100,11FF]. However, there are some jamos that are not encoded in precomposed forms. This is NOT really an issue because they can be represented with sequences of Jamos. Nonetheless, DPRK (North Korea) submitted a proposal to ISO JTC1/SC2/WG2 to add 14 Jamos (consonants and vowels) and one of them was the compound vowel I had found.
Korean Microsoft Word XP also supports far more Hangul Jamos than encoded as precomposed in Unicode 3.1. According to SUNG Seuk Soo, it supports 125 intial consonants, 95 medial vowels and 141 final consonants out of which MS Word can make about 1.5 million syllables. When exporting to html format, MS Word represents Jamos not encoded as of Unicode 3.1 by combining two or more constituent Jamos. For instance, UYEO discussed here is represented as U+116E(ᅮ) followed by U+1167(ᅧ). MS IE 6.0 with appropriate fonts and a version of Uniscribe shipped with Korean MS Office XP and/or MS Windows XP can render this (ᅮᅧ) as UYEO or this (꾸ᅧᆻ) as KKUYEOSS.
In this page, I'll show how that particular vowel UYEO is used in modern Korean.
The vowel UYEO shows up when two adjaecent syllables with 'ㅟ' and 'ㅓ' are shortened/contracted to form a single syllable. For instance, '바뀌었다' is frequently shortened/contracted to '바다'.
Recently I came across UYEO in LEE Sang-Oak's article Graphical Ingenuity in the Korean Writing System in The Korean Alphabet (ed. by Young-Key Kim-Renaud, Univ. of Hawai Press, 1997). Exactly the same example as mine was given for the usage of UYEO in modern Korean. He also cited OYA (ᅩᅣ) as in 도ᅣᆻ다 which is a contracted form of 되았다 in some dialects.