Onji

The term "On" (rarely "Onji") refers to counting phonetic sounds in Japanese poetry. In the Japanese language, the word "on" (音) means "sound". It is used to mean the phonetic units counted in haiku, tanka, and other such poetic forms. Known as "morae" to English-speaking linguists, the modern Japanese term for the linguistic concept is either haku () or mōra ().

Ji (字) is Japanese for "symbol" or "character". The concatenation of the two words "on" and "ji" into "onji" (音字) was used by Meiji era grammarians to mean "phonic character" and was translated into English by Nishi Amane in 1870 as "letter". Since then, the term "onji" has become obsolete in Japan, and only survives in foreign-language discussion of Japanese poetry. Gilbert and Yoneoka call the use of the word "onji" "bizarre and mistaken". It was taken up after a 1978 letter to Frogpond: Journal of the Haiku Society of America decrying the then-current use of the word "jion", which itself appears to have arisen in error. The normal Japanese term in the context of counting sounds in poetry is "on".

Counting on in Japanese poetry is the same as counting characters when the text is transliterated into hiragana. In cases where a hiragana is represented by a pair of symbols each pair (or "digraph" e.g. "kyo" (きょ)) equates to a single on. When viewed this way, the term "ji" ("character") is used in Japanese.

In English-language discussions of Japanese poetry, the more familiar word "syllable" is sometimes used. Although the use of "syllable" is inaccurate, it often happens that the syllable count and the on count match in Japanese-language haiku. The disjunction between syllables and on becomes clearer when counting sounds in English-language versions of Japanese poetic forms, such as haiku in English. An English syllable may contain one, two or three morae and, because English word sounds are not readily representable in hiragana, a single syllable may require many more ji to be transliterated into hiragana.

There is disagreement among linguists as to the definitions of "syllable" and "mora". In contrast, ji (and hence on) is unambiguously defined by reference to hiragana.

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