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The yōon (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,^[1] 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) or に (ni), plus a smaller version of one of the three y kana, ya, yu or yo. For example, 今日 (kyō, "today") is written きょう [kʲoo], using a small version of the よ kana. Contrast this with 器用 (kiyō, "skillful"), which is written きよう [kijoo], with a full-sized よ 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^{[citation needed]}. Although obsolete in modern Japanese, kwa and kwi can still be found in several of the Ryukyuan languages today (e.g. Okinawan), while kwe is formed with the digraph くぇ. Kwa also appears in the Kagoshima dialect. Instead of the kana き, these are formed with the kana for ku, く/ク. Some older transliterations in English follow the earlier pronunciation, e.g. Kwaidan, Kwannon, and such yōon may appear in loanwords e.g. ムジカ・アンティクヮ・ケルン "Musica Antiqua Köln". They were also used to write Hakka in Taiwan under Japanese rule.

Braille
Yōon	Yōon + Dakuten	Yōon + Handakuten	-w-

In Japanese Braille, Yōon is indicated with one of the yōon, yōon+dakuten, or yōon+handakuten prefixes.

Unlike in kana, Braille yōon is prefixed to the -a/-u/-o morae, rather than appending ya, yu or yo to an -i kana, e.g. kyu: きゅ - ki + yu → ⠈⠩ - yōon + ku. Likewise, the -w- morae are indicated by a prefix of the -a/-i/-e/-o morae, rather than an -u mora, e.g. くぁ / くゎ (kwa) = -w- + ka: ⠢⠡.