Consonant and vowel nomenclature

The terms "consonant stem verbs" and "vowel stem verbs" come from a pattern that emerges from studying the actual structure of the words rather than the written representation. When considering the invariant part of the verb (the verb stem), the final phoneme determines the classification of the verb group. If the verb stem's final phoneme:

• is a consonant, then it is a consonant stem verb (pentagrade verb)
• is a vowel, then it is a vowel stem verb (monograde verb)

There are criticisms of the consonant and vowel nomenclature:

1. When pentagrade verbs end with "う" (u), the verb's invariant stem always ends with a vowel, yet is still classified as having a consonant stem. For example, 買う (kau, to buy) has the vowel "a" as the invariant suffix, yet it is still categorized as a "consonant stem verb".
   In these cases, this apparent contradiction is resolved by realizing that the verb's invariant stem ends in the consonant w. The w is normally suppressed, but surfaces in the negative form, as seen in 買わない (kawanai, to not buy). Traditionally these verbs ended in -hu, which is still seen on occasion in historical kana usage, and thus unambiguously ended in h.
   
   
2. When godan verbs end with "つ" (tsu), the verb's invariant stem always ends with an "s" rather than a "t". Since the consonant stem terminology focuses on rōmaji, this could lead to conjugation errors. For example, 待つ (matsu, to wait) in its negative conjugation does not become "待つぁない" (matsanai) as the consonant stem system might have one believe; the correct conjugation is 待たない (matanai, to not wait). The matter is resolved when phonemic notation of "tu" used by Kunrei-shiki romanization is applied instead.
   
   
3. In the case of the past-tense and te forms of conjugation, the 'invariant' stem changes such that the consonant is removed from all godan verbs (except verbs ending in "つ" (tsu) or "す" (su)). This means the defining characteristic of consonant stem verbs cannot be used to define consonant stem verbs for the past-tense or te forms. The true "invariant stem", which is consistent amongst all conjugations, precedes the so-called "invariant consonant".
   
   

Rules of thumb

If a dictionary is unavailable, it becomes difficult to discriminate pentagrade verbs from monograde verbs when they rhyme with 〜ぃる (-iru) or 〜ぇる (-eru). The following heuristics aim to improve the accuracy of naive classification:

• There are far more pentagrade verbs^[2] than monograde verbs.^[3]
• Verbs that do not rhyme with 〜ぃる (-iru) or 〜ぇる (-eru) are pentagrade verbs.

This includes verbs that rhyme with 〜ぁる (-aru), 〜ぅる (-uru) and 〜ぉる (-oru), which are pentagrade verbs.

• The majority of verbs that rhyme with 〜ぃる (-iru) are pentagrade verbs.

248 of the 419 〜ぃる (-iru) verbs [ca. 60%] listed in JMdict are pentagrade verbs.^{[citation needed]}

• The majority of verbs that rhyme with 〜ぇる (-eru) are monograde verbs.

2886 of the 3013 〜ぇる (-eru) verbs [ca. 95%] listed in JMdict are monograde verbs.^{[citation needed]}

Kana and kanji based heuristics for 〜ぃる (-iru) and 〜ぇる (-eru) verbs:

• Verbs written entirely in hiragana are pentagrade verbs. For example, びびる (bibiru, to be surprised) and のめる (nomeru, to fall forward) are pentagrade verbs.
• Kanji verbs with 1 okurigana and 3+ syllables are pentagrade verbs. For example, 契る (chi-gi-ru, to pledge) and 嘲る (a-za-ke-ru, to ridicule) are pentagrade verbs.
• Kanji verbs with 2 okurigana are usually monograde verbs. For example, 起きる (okiru, to get up) and 食べる (taberu, to eat) are monograde verbs.
• Kanji verbs with 2 syllables are inconclusive. For example, 切る (ki-ru) and 見る (mi-ru) are both 2-syllable verbs, yet belong to different categories (pentagrade and monograde, respectively)