Japanese godan and ichidan verbs

The Japanese language has two main types of verbs: godan verbs, or quinquegrade verbs, and ichidan verbs, or unigrade verbs.

Terminology

Categories are important when conjugating Japanese verbs, since conjugation patterns vary according to the verb's category. For example, and belong to different verb categories and therefore follow different conjugation patterns. Most Japanese verbs are allocated into two categories:

Quinquegrade,
Unigrade

Statistically, there are about twice as many quinquegrade verbs than unigrade verbs.
Classical Japanese had more verb groups, such as bigrade and quadrigrade, which are archaic in Modern Japanese.
The word grade in quinquegrade and unigrade is translated from. In grammar, dan is a synonym for and opposite to. The translations for dan/''retsu and gyō vary, either of them can be translated as "row" or "column" depending on how the gojūon table is laid out, but the distinction is simply that gyō is named after consonants, as ka-row, while dan/retsu is named after vowels, as in i''-column or i-grade. The a-row consists the kana,,,,, which differ only by vowels; while the a-grade consists of the kana,,,,,,,,,,,,,,, which differ only by consonants.
The quinquegrade class consists of verbs whose inflection forms make use of all five grades, or five vowels. For example, the inflection forms of the verb are /,,, and. These verbs developed from the earlier quadrigrade class, after a historical sound change that turned such forms like into and resulted in an additional vowel.
The unigrade class consists of verbs that occupy only one grade, or one vowel. Dictionaries may further divide this class into "upper unigrade") if the vowel is the "upper" i, and "lower unigrade") if the vowel is the "lower" e. The verb, whose inflection forms are,, and /, is an example of an "upper unigrade" verb, and the verb is a "lower unigrade" verb. Some unigrade verbs evolved from earlier forms of bigrade verbs.
Dictionaries often list ancestral forms of modern verbs as well as their classes. Thus, the entry for kaku may include a note like （動カ五［四］）, which means "verb, ka-row, quinquegrade, formerly quadrigrade"); while the entry for may include （動カ下一）カ下二う・く, which means "verb, ka-row, lower unigrade, lower bigrade equivalent in Classical Japanese is ".
Note that the choices of prefixes in these English terms by some authors are rather inconsistent: while mono- and penta- are Greek, uni-, bi-, quadri- and quinque- are Latin. The word grade is actually Latin, not Greek in origin. While many authors use bigrade and quadrigrade consistently, they also use any combination of unigrade, monograde, quinquegrade, quinquigrade and pentagrade. Some dispense with quinquegrade altogether and prefer only quadrigrade even for modern Japanese. Plain English alternatives to "unigrade", "bigrade", "quadrigrade" and "quinquegrade" include "one-grade", "two-grade", "four-grade" and "five-grade"; "one row", "two row" and "four row"; "one-step", "two-step", "four-step" and "five-step"; or "one-vowel", "two-vowel" and "four-vowel".
Some Western analyses refer to "quinquegrade" verbs as "consonant-stem" verbs. Such analyses may represent the root form of the verb 書く as kak-, emphasizing the unchanging consonant k. "Unigrade" verbs are then referred to as "vowel-stem" verbs, for example 見る analyzed as mi-.
Historically, the so-called "Japanese irregular verbs verbs and were sometimes known as "trigrade", 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 the gojūon table.
In the table above, the verb uses kana from all 5 rows of the gojūon table in its inflectional suffix—,,, and —amongst its conjugations. Thus, it is classified as a "class-5" verb. Meanwhile, the verbs and each use kana from only 1 row of the gojūon table in their verb-stem's suffix— and respectively. Thus, they are classified as a "class-1" verbs.

''Godan'' vs ''yodan''

All modern godan verbs are derived from historical yodan verbs. The distinction between these two classes relies solely on the interaction between the and the "tentative" auxiliary . Consider the verb :

kaka- + -mu → kakamu → kakau → kakō

The shift of vowels from au to ō was regular and expansive during Late Middle Japanese, and it practically introduced an additional to the inflectional forms of yodan verbs:
The term is a fairly modern coinage. During the time when modern kana usage was being adopted to write modern Japanese in place of historical kana usage, one of the changes concerned how such a form as kakō should be spelt. The modern spelling かこう was proposed along with godan as the name for the modernized yodan class. Traditionalist grammarians, on the other hand, would insist on such spelling as かかう to reflect the historical pronunciation kakau, and on the modern pronunciation being inferred from such spelling. Some argued that a single interaction with the auxiliary u did not justify creating an entire new grammatical class, given that the mizenkei does not involve a vowel shift with any other auxiliary:

kaka- + -nai → kakanai
kaka- + -nu → kakanu
kaka- + -n → kakan
kaka- + -zu → kakazu

Moreover, the auxiliary -ta and the particle -te also notably alter the ren'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 term yodan or the term godan at all, despite occurring in both these supposedly different inflections. This means that exceptional interactions with auxiliaries and particles like these ought not to be the basis for naming verb classes.
Obviously, the spelling reform took place and the term godan became mainstream. Historical kana usage is now reserved only for the writing of classical Japanese, and yodan verbs are largely considered a classical Japanese class while godan verbs make up a fundamental part of modern Japanese.

Japanese language education

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

	Quinquegrade verbs	Unigrade verbs	Irregular verbs	Example literature
Common terminology	Group 1	Group 2	Group 3	A Dictionary of Basic Japanese Grammar
Common terminology	Group I	Group II	Group III	A Dictionary of Basic Japanese Grammar
Uncommon terminology			Irregular verbs	GENKI
Rare terminology	Consonant stem verbals	Vowel stem verbals	-	Japanese: The Spoken Language

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

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 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
is a vowel, then it is a vowel stem verb

There are criticisms of the consonant and vowel nomenclature:

When quinquegrade verbs end with, the verb's invariant stem always ends with a vowel, yet is still classified as having a consonant stem. For example, 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 consonant w. The w is normally suppressed, but surfaces in the negative form, as seen in. Traditionally these verbs ended in -hu, which is still seen on occasion in historical kana usage, and thus unambiguously ended in h.

When godan verbs end with, 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, in its negative conjugation does not become as the consonant stem system might have one believe; the correct conjugation is. The matter is resolved when phonemic notation of "tu" used by Kunrei-shiki romanization is applied instead.

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. 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".

Verb classification

Classifying verbs is simple in theory:

Take the verb in its plain, negative form. The result will be: verb-stem +
If the last character of the verb-stem :

Negative verb	Last character of verb stem	Rhymes with	Group
		→	Quinquegrade verb
		→	Quinquegrade verb
		→	Unigrade verb
		→	Unigrade verb

This classification system works for all Japanese verbs, with three exceptions: is a quinquegrade verb, and both and are instead classified as irregular verbs.