nidan conjugation; bigrade conjugation
A classical Japanese verb conjugation class with two base vowels (e.g., 上二段 and 下二段). Used in historical grammar; modern equivalents are 上一段 and 下一段.
Nidan conjugation is one of the conjugations studied in classical grammar.
The bigrade conjugation is the origin of the modern monograde conjugation.
Modern monograde conjugation; evolved from 二段活用. 一段 verbs have a single base vowel.
Classical quadrigrade conjugation; another major classical verb class, which became modern 五段活用.
From 二段 (two grades) + 活用 (conjugation), referring to the two distinct vowel stems in classical Japanese verb inflection.