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Excerpt from Inuyasha wiki: "In the English adaptations of InuYasha, Tessaiga is called Tetsusaiga (or Tetsaiga) due to an error in translation in the early days of the manga when the tiny っ (tsu) kana (which indicates a glottal stop, or a doubling of the consonant that follows it) was misread as a full-sized つ (tsu) kana, altering the romanization and pronunciation of the word. Both Tetsusaiga and Tessaiga can come from the kanji; the proper spelling depends on how the name is pronounced. The proper romanization is Tessaiga. By the time Viz, the company that translates the anime and manga, realized the error, it was too late."