köken: From classical 下二段 verb 解く (toku). Development Natural progression from Old and Classical Japanese 下(しも)二(に)段(だん)活(かつ)用(よう) (shimo nidan katsuyō, “lower bigrade conjugation”) verbs ending in -u to modern Japanese 下(しも)一(いち)段(だん)活(かつ)用(よう) (shimo ichidan katsuyō, “lower monograde conjugation”) verbs ending in -eru.