Onbin (Japanese onbin (音便) "euphony") is a set of sound changes that occurred in Early Middle Japanese around the end of the eighth century to the beginning of the tenth century,[1] first attested in written texts of the Heian period.[2] Onbin changes affected certain consonant-vowel sequences in non-word-initial position, causing them to become replaced with either a single vowel sound (as in oite, from earlier /okite/) or a single consonant sound (as in shinde, from earlier /sinite/).[3] (In some cases, this also caused a change in the pronunciation of the preceding vowel or following consonant, as in the development of original /te/ to /de/ in shinde.) Onbin played a role in diversifying the syllable structure of native Japanese words by creating heavy syllables that ended in two vowels or in a vowel followed by a consonant.[4]
Historical onbin changes did not occur systematically,[5] and some sequences could yield multiple outcomes. The non-deterministic nature of the historical sound changes is exemplified by doublets showing different outcomes of the same original form, such as komichi (without onbin) versus kōji (with u-onbin) from original /komiti/, or akindo (with N-onbin) versus akyūdo (with u-onbin) from original /akibito/. However, some onbin changes have come to be grammaticalized in the conjugation of Japanese verbs: as a result, certain verbs systematically display an "onbin stem" before certain suffixes in standard modern Japanese. The formation of these onbin stems varies between dialects.