What does YOON stand for? 

Yōon

The yō-on (Japanese: 拗音 (ようおん)), also written as yōon, is a feature of the Japanese language in which a mora is formed with an added [j] sound, i.e., palatalized, or (more rarely in the modern language) with an added [w] sound, i.e. labialized. Yōon are represented in hiragana using a kana ending in i, such as き (ki), plus a smaller version of one of the three y kana, ya, yu or yo. For example, kyō, "today" (今日), is written きょう [kʲoo], using a small version of the yo kana, よ. Contrast this with kiyō, "skillful" (器用), which is written きよう [kijoo], with a full-sized yo kana. In historical kana orthography, yōon were not distinguished with the smaller kana, and had to be determined by context. In earlier stages of Japanese, and in certain dialects, yōon can also be formed with the kana wa, wi, we, and wo; for example, くゎ/クヮ kwa,　くゐ/クヰ kwi, くゑ/クヱ kwe,　くを/クヲ kwo. Although obsolete in modern Japanese, kwa and kwi can still be found in several of the Ryukyuan languages today, while kwe is formed with the digraph くぇ. Instead of the kana き, these are formed with the kana for ku, く/ク.

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