 The difference between 'upper' and 'lower' bigrade or monograde verbs is whether the vowel at the end of the root is "i" or "e".There is only one 'lower' monograde verb, "ke"- 'kick', which was a 'lower' bigrade verb "kuwe"- in Old Japanese, and which changed classes again in later Japanese to become quadrigrade (Modern Japanese "ker"-).The difference between bigrade and monograde is whether, in the conclusive, attributive and realis, the initial "u" of the ending elides the vowel of the root or the vowel of the roots elides the initial "u" of the ending.