Alternative kana letterforms equivalent to standard kana characters. This is a legacy of man'yōgana, where many different kanji could be used to represent the same syllable; as these became simplified into cursive forms, hentaigana were the result. Hentaigana were used more or less interchangeably with their equivalent hiragana on an ad hoc, individual basis until 1900, when the hiragana syllabary was standardized to one character per mora. (from Wikipedia)
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