The あ行 (a-gyō "a-row") consists the kana あ (a), い (i), う (u), え (e), お (o), which differ only by vowels; while the あ段 (a-dan "a-grade") consists of the kana あ (a), か (ka), が (ga), さ (sa), ざ (za), た (ta), だ (da), な (na), は (ha), ば (ba), ぱ (pa), ま (ma), や (ya), ら (ra), わ (wa), which differ only by consonants.
The 五段 (godan "pentagrade" or "quinquigrade") class consists of verbs whose inflection forms make use of all five grades, or five vowels.
These verbs developed from the earlier 四段 (yodan "quadrigrade") class, after a historical sound change that turned such form as 書かむ (kakamu) into 書こう (kakō) and resulted in an additional vowel (see Late Middle Japanese).
Thus, the entry for 書く may include a note like （動カ五［四］）,[8] which means "verb, ka-row, pentagrade, formerly quadrigrade"); while the entry for 受ける (ukeru "to take") may include （動カ下一）[文]カ下二 う・く,[8] which means "verb, ka-row, lower monograde, lower bigrade equivalent in Classical Japanese is 受く".
Note that the choices of prefixes in these English terms by some authors are rather inconsistent: while mono- and penta- are Greek, bi-, quadri- and quinqui- are Latin.
Meanwhile, the verbs 見る (to see) and 止める (to stop) each use kana from only 1 row of the gojūon table in their verb-stem's suffix—み (mi) and め (me) respectively.
[15] The distinction between these two classes relies solely on the interaction between the mizenkei (未然形) and the "tentative" (推量, suiryō) auxiliary u (う) (historically, mu (む)).
Consider the verb kaku (書く, "to write"): The shift of vowels from au to ō was regular and expansive during Late Middle Japanese, and it practically introduced an additional dan (段, in other words, vowel) to the inflectional forms of yodan verbs: The term godan (五段) is a fairly modern coinage.
Similarly, (う) or (る) may be notated beside verbs in literature adopting the "う-verbs / る-verbs" terminology.
[16] In some Japanese dictionaries, the readings of conjugable words may have the stem and the inflectional suffix separated by a dot (・).
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: 要る (iru, to need), 入る (hairu, to enter), 走る (hashiru, to run), 帰る (kaeru, to return, to go home), 切る (kiru, to cut), 知る (shiru, to know), 喋る (shaberu, to talk) Naive strategies, such as this one, tend to misidentify pentagrade verbs ending with る (ru)—specifically, when pentagrade verbs rhyme with 〜ぃる (-iru) or 〜ぇる (-eru).
For example: (Rōmaji) When reading verbs such as these, the correct word meaning can be ascertained through the different kanji or accentuation.