Nihon and Nippon
The Japanese name for Japan, 日本, can be pronounced either Nihon or Nippon. Both readings come from the on’yomi.
日 (nichi) means “sun” or “day”; 本 (hon) means “base” or “origin”. The compound means “base of the sun” or “sunrise” (from a Chinese point of view, the sun rises from Japan); it is of course a source for the popular Western description of Japan as the “Land of the Rising Sun”.
Nichi, in compounds, often loses the final chi and creates a slight pause between the first and second syllables of the compound. When romanised, this pause is represented by a doubling of the first consonant of the second syllable; thus nichi 日 plus kō 光 (light) is written and pronounced nikkō, meaning sunlight. The initial consonant of non-initial syllables in compounds often becomes voiced in Japanese, in a process called rendaku. Non-initial hon in compounds thus often changes to bon or pon. There are therefore two possible pronunciations for 日本: Nihon or Nippon. While both pronunciations are correct, Nippon is frequently preferred for official purposes, including money, stamps, and international sporting events, as well as the Nippon koku, literally the “State of Japan” (日本国).
Other than this, there seem to be no fixed rules for choosing one pronunciation over the other; in some cases one form is simply more common. For example, Japanese speakers generally call their language Nihongo; Nippongo, while possible, is rare. In other cases, uses are variable. The name for the Bank of Japan (日本銀行), for example, is given as NIPPON GINKO on banknotes, but often referred to (in the media, for example) as Nihon Ginkō.
Nippon is used always or most often in the following constructions:
Nippon-koku kenpō (Constitution of Japan)
Ganbare Nippon! (A sporting cheer used at international sporting events, roughly, ‘do your best, Japan!’)
Zen Nippon Kūyu (All Nippon Airways)
Nippon-bashi (日本橋) (a shopping district in Osaka)
Nihon is used always or most often in the following constructions:
Nihon-bashi (日本橋) (a bridge in Tokyo)
Nihon Daigaku (Nihon University)
Nihon-go (Japanese language)
Nihon-jin (Japanese people)
Nihon-kai (Sea of Japan)
Nihon Kōkū (Japan Airlines)
Nihon-shoki (an old history book, never Nippon shoki)