A historical change in which, as to the attributive and classical imperfective forms of upper and lower bigraded conjugations, the ending "-uru," "-ure" became "-iru," "-ire"respectively and finally developed into inflected forms similar to those of upper and lower bigraded conjugations is called a change of two-tier conjugation verbs into one-tier.
下二段活用・上二段活用の連体形・已然形において、-uru・-ureの語尾が-eru・-ere、-iru・-ireの形になってそれぞれ下一段、上一段と同じような活用形になる史的変化のことを二段活用の一段化という。