↑Prior to the e/ye merger in the mid-Heian period, a different character (𛀀) was used in position e.
↑ 2.02.1Theoretical combinations yi and wu are unused. Some katakana were invented for them by linguists in the Edo and Meiji periods in order to fill out the table, but they were never actually used in normal writing.
↑The combination ye existed prior to the mid-Heian period and was represented in very early katakana, but has been extinct for over a thousand years, having merged with e in the 10th century. The ye katakana (エ) was adopted for e (displacing 𛀀, the character originally used for e); the alternate katakana 𛄡 was invented for ye in the Meiji period for use in representations of Old and Early Classical Japanese so as to avoid confusion with the modern use of エ for e.
↑ 4.04.14.2The characters in positions wi and we are obsolete in modern Japanese, and have been replaced by イ (i) and エ (e). The character wo, in practice normally pronounced o, is preserved in only one use: as a particle. This is normally written in hiragana (を), so katakana ヲ sees only limited use. See Gojūon and the articles on each character for details.
↑ 5.05.15.25.35.4The ヂ (di) and ヅ (du) kana (often romanised as ji and zu) are primarily used for etymologic spelling, when the unvoiced equivalents チ (ti) and ツ (tu) (usually romanised as chi and tsu) undergo a sound change (rendaku) and become voiced when they occur in the middle of a compound word. In other cases, the identically-pronounced ジ (ji) and ズ (zu) are used instead. ヂ (di) and ヅ (du) can never begin a word, and they are not common in katakana, since the concept of rendaku does not apply to transcribed foreign words, one of the major uses of katakana.
