Nihon-shiki romanization

Nihon-shiki (日本式ローマ字, romanized as Nihonsiki in the system itself) is a romanization system for transliterating the Japanese language into the Latin alphabet. Among the major romanization systems for Japanese, it is the most regular one and has an almost one-to-one relation to the kana writing system.

History
It was invented by physicist Aikitsu Tanakadate (田中館 愛橘) in 1885, with the intention to replace the Hepburn system of romanization. Tanakadate's intention was to replace the traditional kanji and kana system of writing Japanese completely by a romanized system, which he felt would make it easier for Japan 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.

Nihon-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-shiki is nearly identical to Nihon-shiki, but it merges 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, and gwa/ga ぐゎ/が, whose pronunciations in Modern Standard Japanese are now identical. For example, the word かなづかい, rendered kanadukai in Nihon-shiki, is pronounced as kanazukai in modern Japanese, and is romanized as such in Kunrei. The International Organization for Standardization has standardized Kunrei-shiki, under ISO 3602. The JSL system, which is intended for use instructing foreign students of Japanese, is also based on Nihon-shiki. However, some Japanese-speakers still distinguish di from zi and du from zu and so Nihon-shiki spelling is not entirely obsolete.

Nihon-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 (almost) lossless ("round trip") mapping, but the standard does not mandate the precise spellings needed to distinguish ô 王/おう, ou 追う/おう and oo 大/おお. (See the hiragana article for more details.)