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Nihon-shiki or Nippon-shiki Rōmaji (Japanese: 日本式ローマ字, "Japan-style"; romanized as Nihon-siki or Nippon-siki in Nippon-shiki itself) is a romanization system for transliterating the Japanese language into the Latin alphabet. In discussion about romaji, it is abbreviated as Nihon-shiki or Nippon-shiki. Among the major romanization systems for Japanese, Nippon-shiki is the most regular, and has a one-to-one relation to the kana writing systems. In practice, Nippon-shiki has been largely supplanted by Hepburn romanization.
It was invented by physicist Aikitsu Tanakadate (田中館 愛橘 Tanakadate Aikitsu, 1856-1952) in 1885, with the intention to replace the Hepburn system of romanization. Tanakadate's intention was to completely replace the traditional kanji and kana system of writing Japanese with a romanized system, which he felt would make it easier for Japanese people to compete with Western countries. Since the system was intended for Japanese people to use to write their own language, it is much more regular than Hepburn romanization, and, unlike Hepburn's system, it makes no effort to make itself easier to pronounce for English speakers.
Nippon-shiki was followed by Kunrei-shiki, which was adopted in 1937 after a political debate over whether Nihon-shiki or Hepburn-shiki should be used by the Japanese government. Kunrei is otherwise nearly identical, but it merges the syllable pairs di/zi ぢ/じ, du/zu づ/ず, dya/zya ぢゃ/じゃ, dyu/zyu ぢゅ/じゅ, dyo/zyo ぢょ/じょ, wi/i ゐ/い, we/e ゑ/え, kwa/ka くゎ/か, gwa/ga ぐゎ/が, whose pronunciations in modern standard Japanese have become identical. For example, the word かなづかい, rendered kanadukai in Nippon-shiki, is pronounced as kanazukai in modern Japanese and romanized as such in Kunrei.
Nippon-shiki is considered the most regular of the romanization systems for the Japanese language, because it maintains a strict "one kana, two letters" form. Because it has unique forms corresponding to each of the respective pairs of kana homophones listed above, it is the only formal system of romanization that can allow lossless ("round trip") mapping, although the standard does not mandate the precise spellings needed to distinguish ô 王/おう, ou 追う/おう and oo おお, so it is not completely lossless. (See the hiragana article for more details.)
Nippon-shiki has been established by the International Organization for Standardization in the ISO 3602 strict form. The JSL system, which is intended for use instructing foreign students of Japanese, is also based on Nippon-shiki.
Nipponsiki—ISO 3602 Strict 
|あ/ア a||い/イ i||う/ウ u||え/エ e||お/オ o||(ya)||(yu)||(yo)|
|か/カ ka||き/キ ki||く/ク ku||け/ケ ke||こ/コ ko||きゃ/キャ kya||きゅ/キュ kyu||きょ/キョ kyo|
|さ/サ sa||し/シ si||す/ス su||せ/セ se||そ/ソ so||しゃ/シャ sya||しゅ/シュ syu||しょ/ショ syo|
|た/タ ta||ち/チ ti||つ/ツ tu||て/テ te||と/ト to||ちゃ/チャ tya||ちゅ/チュ tyu||ちょ/チョ tyo|
|な/ナ na||に/ニ ni||ぬ/ヌ nu||ね/ネ ne||の/ノ no||にゃ/ニャ nya||にゅ/ニュ nyu||にょ/ニョ nyo|
|は/ハ ha||ひ/ヒ hi||ふ/フ hu||へ/ヘ he||ほ/ホ ho||ひゃ/ヒャ hya||ひゅ/ヒュ hyu||ひょ/ヒョ hyo|
|ま/マ ma||み/ミ mi||む/ム mu||め/メ me||も/モ mo||みゃ/ミャ mya||みゅ/ミュ myu||みょ/ミョ myo|
|や/ヤ ya||ゆ/ユ yu||よ/ヨ yo|
|ら/ラ ra||り/リ ri||る/ル ru||れ/レ re||ろ/ロ ro||りゃ/リャ rya||りゅ/リュ ryu||りょ/リョ ryo|
|わ/ワ wa||ゐ/ヰ wi||ゑ/ヱ we||を/ヲ wo|
|voiced sounds (dakuten)|
|が/ガ ga||ぎ/ギ gi||ぐ/グ gu||げ/ゲ ge||ご/ゴ go||ぎゃ/ギャ gya||ぎゅ/ギュ gyu||ぎょ/ギョ gyo|
|ざ/ザ za||じ/ジ zi||ず/ズ zu||ぜ/ゼ ze||ぞ/ゾ zo||じゃ/ジャ zya||じゅ/ジュ zyu||じょ/ジョ zyo|
|だ/ダ da||ぢ/ヂ di||づ/ヅ du||で/デ de||ど/ド do||ぢゃ/ヂャ dya||ぢゅ/ヂュ dyu||ぢょ/ヂョ dyo|
|ば/バ ba||び/ビ bi||ぶ/ブ bu||べ/ベ be||ぼ/ボ bo||びゃ/ビャ bya||びゅ/ビュ byu||びょ/ビョ byo|
|ぱ/パ pa||ぴ/ピ pi||ぷ/プ pu||ぺ/ペ pe||ぽ/ポ po||ぴゃ/ピャ pya||ぴゅ/ピュ pyu||ぴょ/ピョ pyo|
- Letters in red are obsolete in modern Japanese.
- When he へ is used as a particle it is written he not e (Kunrei-shiki/Hepburn).
- When ha は is used as a particle it is written ha not wa.
- When wo を is used as a particle it is written wo not o.
- Long vowels are indicated by a circumflex, for example long o is written ô, unlike Hepburn, which uses a macron.
- Syllabic n ん is written as n before consonants but as n' before vowels and y.
- Geminate consonants are marked by doubling the consonant following the sokuon, っ, without exception.
See also 
- Gottlieb, Nanette. "The Rōmaji movement in Japan." Journal of the Royal Asiatic Society (Third Series). January 2010. Volume 20, Issue 1. p. 75-88. Published online on November 30, 2009. Available at Cambridge Journals. DOI doi:10.1017/S1356186309990320.
- Kent, Allen, Harold Lancour, and Jay Elwood Daily (Executive Editors). Encyclopedia of Library and Information Science Volume 21. CRC Press, April 1, 1978. ISBN 0824720210, 9780824720216.
- Nihongo Daihakubutsukan (日本語大博物館), author: Kida, Jun'ichirō (紀田順一郎 Kida Jun'ichirō), publisher: Just System (ジャストシステム, Jasuto Shisutemu) ISBN 4-88309-046-9 (in Japanese), chapter 6.
- Gottlieb, p. 78
- Kent, et al. "Oriental Literature and Bibliography." p. 155.