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TheJapanese language has two main types ofverbs:godan verbs, orquinquegrade verbs (五段動詞,godan-dōshi), andichidan verbs, orunigrade verbs (一段動詞,ichidan-dōshi).

Categories are important when conjugating Japanese verbs, sinceconjugation patterns vary according to the verb's category. For example,切る (kiru) and見る (miru) belong to different verb categories (quinquegrade and unigrade, respectively) and therefore follow different conjugation patterns. Most Japanese verbs are allocated into two categories:

1. Quinquegrade,^[1][2][3]quinquigrade,^[4][5][6]quinquagrade^[7] orpentagrade^[8][9] verbs (五段動詞,godan-dōshi)
2. Unigrade^{[10][11][12][13][8][5][14]} ormonograde^{[2][15][7][16][17][6][18][9]} verbs (一段動詞,ichidan-dōshi)

Statistically, there are about twice as many quinquegrade verbs^[19] than unigrade verbs.^[20]

Classical Japanese had more verb groups, such asbigrade^{[21][12][13][2][22][23][14][24][25][18]} verbs (二段動詞,nidan-dōshi) andquadrigrade^{[21][12][13][15][23][14][24][25][26]} verbs (四段動詞,yodan-dōshi), which are archaic inModern Japanese.

The wordgrade inquinquegrade andunigrade is translated fromdan (段).^[a] In grammar,dan is a synonym forretsu (列)^[27] and opposite togyō (行). The translations fordan/retsu and gyō vary, either of them can be translated as "row" or "column", but the distinction is simply thatgyō is named after consonants, aska-row (か行,ka-gyō), whiledan/retsu is named after vowels, as ini-column (い列,i-retsu) ori-grade (い段,i-dan). Thea-row (あ行,a-gyō) consists the kanaa (あ),i (い),u (う),e (え),o (お), which differ only by vowels; while thea-grade (あ段,a-dan) consists of the kanaa (あ),ka (か),ga (が),sa (さ),za (ざ),ta (た),da (だ),na (な),ha (は),ba (ば),pa (ぱ),ma (ま),ya (や),ra (ら),wa (わ), which differ only by consonants.

The quinquegrade (五段,godan) class consists of verbs whose inflection forms make use of all five grades, or five vowels. For example, the inflection forms of the verbkaku (書く; "to write; to draw",ka-row) arekaka (書か)/kako (書こ),kaki (書き),kaku (書く), andkake (書け). These verbs developed from the earlier quadrigrade (四段,yodan) class, after a historical sound change that turned such forms likekakamu (書かむ) intokakō (書こう) and resulted in an additional vowel (seeLate Middle Japanese).

The unigrade (一段,ichidan) class consists of verbs that occupy only one grade, or one vowel. Dictionaries^{[27][28][29][30]} may further divide this class into "upper unigrade" (上一段,kamiichidan)) if the vowel is the "upper"i, and "lower unigrade" (下一段,shimoichidan))^[22] if the vowel is the "lower"e. The verbmiru (見る; "to look",ma-row), whose inflection forms aremi (見),miru (見る),mire (見れ) andmiro (見ろ)/miyo (見よ), is an example of an "upper unigrade" verb, and the verberu (得る; "to earn",a-row) is a "lower unigrade" verb. Some unigrade verbs evolved from earlier forms of bigrade verbs (upper bigrade (上二段,kaminidan) (sometimes also known as "middle bigrade" (中二段,nakanidan)) and lower bigrade (下二段,shimonidan)).

Dictionaries often list ancestral forms of modern verbs as well as their classes.^{[27][28][29][30]} Thus, the entry forkaku may include a note like（動カ五［四］）,^[27] which means "verb,ka-row, quinquegrade, formerly quadrigrade"); while the entry forukeru (受ける; "to take") may include（動カ下一）[文]カ下二 う・く,^[27] which means "verb,ka-row, lower unigrade, lower bigrade equivalent inClassical Japanese isuku (受く)".

Note that the choices of prefixes in these English terms by some authors are rather inconsistent: whilemono- andpenta- are Greek,uni-,bi-,quadri- andquinque- are Latin (seeNumeral prefix § Table of number prefixes in English). The wordgrade is actually Latin, not Greek in origin.^[31] While many authors usebigrade andquadrigrade consistently, they also use any combination ofunigrade,monograde,quinquegrade,quinquigrade andpentagrade. Some dispense with quinquegrade (五段,godan) altogether and prefer only quadrigrade (四段,yodan)^[32] even for modern Japanese (seeGodan vsyodan below). Plain English alternatives to "unigrade", "bigrade", "quadrigrade" and "quinquegrade" include "one-grade", "two-grade", "four-grade" and "five-grade";^[33][34] "one row", "two row" and "four row";^[35] "one-step", "two-step", "four-step" and "five-step";^{[32][12][36][b]} or "one-vowel", "two-vowel" and "four-vowel".^[32]

Some Western analyses refer to "quinquegrade" verbs as "consonant-stem" verbs. Such analyses may represent the root form of the verb書く askak-, emphasizing the unchanging consonantk. "Unigrade" verbs are then referred to as "vowel-stem" verbs, for example見る analyzed asmi-.^[37]

Historically, the so-called "irregular" (変格,henkaku) verbssuru (する) andkuru (来る) were sometimes known as "trigrade" (三段,sandan),^[38][39] given that their forms contain three out of the five vowels of Japanese.

Here is a visualization that compares various verb conjugations to an extracted column of thegojūon table.

In the table above, the verb読む(yomu, to read) uses kana from all5 rows of the gojūon table in its inflectional suffix—ま (ma),み (mi),む (mu),め (me) andも (mo)—amongst its conjugations. Thus, it is classified as a "class-5" (or more formally "quinquegrade") verb. Meanwhile, the verbs見る (to see) and止める (to stop) each use kana from only1 row of the gojūon table in their verb-stem's suffix—み (mi) andめ (me) respectively. Thus, they are classified as a "class-1" (or more formally "unigrade") verbs.(See alsoJapanese verb conjugations.)

All moderngodan verbs are derived from historicalyodan verbs.^[40] The distinction between these two classes relies solely on the interaction between themizenkei (未然形) and the "tentative" (推量,suiryō) auxiliaryu (う) (historically,mu (む)). Consider the verbkaku (書く; "to write"):

• kaka- +-mu →kakamu (historical form) →kakau (historical form) →kakō (modern form)

The shift of vowels fromau toō was regular and expansive duringLate Middle Japanese, and it practically introduced an additionaldan (段; in other words, vowel) to the inflectional forms ofyodan verbs:

The termgodan (五段) is a fairly modern coinage. During the time whenmodern kana usage was being adopted to write modern Japanese (口語,kōgo;lit. 'spoken language') in place ofhistorical kana usage, one of the changes concerned how such a form askakō should be spelt. The modern spellingかこう was proposed along withgodan as the name for the modernizedyodan class. Traditionalist grammarians, on the other hand, would insist on such spelling asかかう to reflect the historical pronunciationkakau, and on the modern pronunciation being inferred from such spelling. Some argued that a single interaction with the auxiliaryu did not justify creating an entire new grammatical class, given that themizenkei does not involve a vowel shift with any other auxiliary:^[40]

• kaka- +-nai →kakanai
• kaka- +-nu →kakanu
• kaka- +-n^[c] →kakan
• kaka- +-zu →kakazu

Moreover, the auxiliary-ta and the particle-te also notably alter theren'yōkei:

• kaki +-ta/-te →kaita/te
• omoi +-ta/-te →omotta/te
• yomi +-ta/-te →yonda/de

Yet, such alterations are not reflected by either the termyodan or the termgodan at all, despite occurring in both these supposedly different inflections (although inclassical Japanese (文語,bungo;lit. 'written language'), these alterations in pronunciation must be inferred from the spellings). This means that exceptional interactions with auxiliaries and particles like these ought not to be the basis for naming verb classes.[40]

Obviously, the spelling reform took place and the termgodan became mainstream. Historical kana usage is now reserved only for the writing of classical Japanese, andyodan verbs are largely considered a classical Japanese class whilegodan verbs make up a fundamental part of modern Japanese.

WithinJapanese language education, various terminologies are used in lieu of the Japanese nomenclature for "quinquegrade" and "unigrade" verbs.

In literature adopting the "Group I / II / III" terminology, the terms (I), (II) or (III) may be notated beside verbs.Similarly, (う) or (る) may be notated beside verbs in literature adopting the "う-verbs /る-verbs" terminology.

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 (theverb stem), the final phoneme determines the classification of the verb group. If the verb stem's final phoneme:

• is a consonant, then it is aconsonant stem verb (quinquegrade verb)
• is a vowel, then it is avowel stem verb (unigrade verb)

There are criticisms of the consonant and vowel nomenclature:

1. When quinquegrade 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 expection is resolved by realizing that the verb's invariant stem ends in the consonantw. Thew 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 inhistorical kana usage, and thus unambiguously ended inh.
   
   
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 doesnot 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 byKunrei-shiki romanization is applied instead.
   
   
3. In the case of thepast-tense andte 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 orte forms. The true "invariant stem", which is consistent amongst all conjugations, precedes the so-called "invariant consonant".
   
   

Classifying verbs is simple in theory:

1. Take the verb in its plain,negative form. The result will be:verb-stem +ない (nai)
2. If thelast character of theverb-stem (ignoring the "ない"):

• rhymes with〜ぁ (-a), then it is aquinquegrade verb
• rhymes with〜ぃ (-i) or〜ぇ (-e), then it is aunigrade verb

This classification system works for all Japanese verbs, with three exceptions:ある (aru) is a quinquegrade verb, and bothしない (shinai) andこない (konai) are instead classified asirregular verbs.^[41]

In some Japanese dictionaries, the readings of conjugable words may have the stem and the inflectional suffix separated by a dot (・). For example, the adjective赤い (akai, red) may be written asあか・い (aka·i) to separate the static prefix from the dynamic suffix.

This system also describes the verb group classification: in quinquegrade verbs, the dot is placed before the last kana; in unigrade verbs, the dot is placed before the last 2 kana (except for 2-kana unigrade verbs, which have no dot).

However, regardless of the dot's position, the inflectional suffix is always the last kana of any unigrade verb.

A caveat of accurately classifying verb groups is that you must have pre-existing knowledge of the verb's negative form. In practice, people tend to learn the verb's plain form first. As such, Japanese language educators usually teach strategies for naive verb classification. Whilst such strategies are not comprehensive, they generally remain useful in the context of regular daily conversations that language beginners will likely encounter. Here is one such strategy:

Naive strategies, such as this one, tend to misidentify quinquegrade verbs ending withる (ru)—specifically, when quinquegrade verbs rhyme with〜ぃる (-iru) or〜ぇる (-eru). Therefore, when a unigrade verb is concluded from a naive strategy, it is more efficient to confirm the verb's classification in a dictionary. However, there are other rules-of-thumb to more accurately discriminate such verbs.

If a dictionary is unavailable, it becomes difficult to discriminate quinquegrade verbs from unigrade 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 quinquegrade verbs^[19] than unigrade verbs.^[20]
• Verbs that donot rhyme with〜ぃる (-iru) or〜ぇる (-eru) arequinquegrade verbs.

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

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

70 of the 112〜ぃる (-iru) common verbs [ca. 62%] listed in JMdict are unigrade verbs.^[d]

• The vast majority of verbs that rhyme with〜ぇる (-eru) areunigrade verbs.

485 of the 514〜ぇる (-eru) common verbs [ca. 94%] listed in JMdict are unigrade verbs.^[e]

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

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

There are many quinquegrade verbs which may be mistaken for being unigrade verbs in some cases(see§ Naive verb classification, above). On the surface, this may seem like a problem that only affects conjugation patterns, since quinquegrade verbs and unigrade verbs conjugate differently(See main article:Japanese verb conjugations). However there are many verbs that, despite having the same spelling, have different meanings and belong to different verb groups. For example:

When reading verbs such as these, the correct word meaning can be ascertained through the different kanji or accentuation.(See alsoJapanese pitch accent.) However, ambiguity is usually removed if the verbs have been conjugated somehow, because different word groups conjugate with slightly varying pronunciations. For example:

Since there are so many quinquegrade verbs that resemble unigrade verbs, it is impractical to create or memorize an exhaustive list of words.

• Japanese verb conjugation
• Honorific speech in Japanese
• Japanese adjectives
• Japanese particles
• Japanese grammar
• 五段活用

• Aoki, Hirofumi (2024). "Late Middle Japanese grammar". In Frellesvig, Bjarke; Kinsui, Satoshi (eds.).Handbook of Historical Japanese Linguistics. Handbooks of Japanese Language and Linguistics. De Gruyter.ISBN 978-1-61451-401-5.
• Frellesvig, Bjarke (2010).A History of the Japanese Language. Cambridge: Cambridge University Press.ISBN 978-0-521-65320-6.
• Iwasaki, Shoichi (2001). "Prose and Poetry of Heian Japanese". In Herring, Susan C.; van Reenen, Pieter; Schøsler, Lene (eds.).Textual Parameters in Older Languages. Current Issues in Linguistic Theory. John Benjamins Publishing Company.ISBN 90-272-3702-6.
• Miyagawa, Shigeru (2 December 2019). "Development of Accusative Case Marker".Structure and Case Marking in Japanese. Syntax and Semantics. Academic Press.ISBN 978-0-12-613522-0.
• Vance, Timothy J. (2022).Irregular Phonological Marking of Japanese Compounds. The Mouton NINJAL Library of Linguistics. De Gruyter.ISBN 978-3-11-075501-5.
• Verbeck, Guido Herman Fridolin (1887).A Synopsis of All the Conjugations of the Japanese Verbs. Yokohama: Kelly & Walsh, Limited.
• Vovin, Alexander Vladimirovich (2020).A Descriptive and Comparative Grammar of Western Old Japanese. Languages of Asia Series (2nd ed.). Brill.ISBN 978-90-04-42211-7.

• Archive of the Usenet newsgroup sci.lang.japan forums, a non-exhaustive list of ~iru and ~eru quinquegrade verbs
• Aeron Buchanan's Japanese Verb Chart, a concise summary of all Japanese verbs conjugations on one sheet of A4

• Japanese grammar
• Japonic verbs

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