Why is 一段 called the 一段?

Question

It's said that 一段 are called 一段 because they keep their stem, but is that really true?

Like, in 終止形, the verb has る, which is ウ段. And then, in 命令形, it has ろ, which is オ段.

How do you explain this?

Answer 1

Its said that 一段 are called 一段 because they keep their stem.

You just answered your own question. 一段 verbs keep their stems intact, while the suffix change to reflect different conjugations. For example:

• 食べる dictionary form
• 食べます respectful form
• 食べない negative form
• 食べません negative-respectful form
• 食べた past tense
• 食べれば if-form
• 食べろ imperative form
• 食べさせる causative form
• 食べられない negative-potential form
• The list goes on and on...

You can see how in each of these forms the stem 食べ never changes. Different endings are added, which happen to include る for dictionary form and ろ for imperative. For 五段, let's see a comparison:

• 歩く dictionary form
• 歩きます respectful form
• 歩かない negative form
• 歩きません negative-respectful form
• 歩いた past tense
• 歩けば if-form
• 歩け imperative form
• 歩かせる causative form
• 歩けない negative-potential form
• The list goes on and on...

While the stem itself is 歩き, the き could change to か き く け or こ depending on which 段 the conjugation needs to be in. There are five in total(かきくけこ) hence the name 五段 verbs.

Answer 2

dvx2718's answer should be canonical. I just add to clarify the point (I think) you are confused about.

Take 切る(五段) and 切れる(下一段) for examples.

In my understanding, 切り¹/切れ¹ are both masu-stem in learners' grammar. Note 切り¹ of 切る is a conjugating part of 切る and 切れ¹ is the NON-conjugating part of 切れる. This may well make you think that れる etc. is part of stem. But (at least in some contexts), this is not the case, as mentioned in this Wikipedia article (which should apply to 下一段 as well with obvious changes).

言語学から言えば、上一段活用の動詞は語幹が母音で終わる母音語幹動詞である。そのため語幹は「る」の前のイ段母音までとされ、それ以後が語尾とされる

So the bolded る・る・れ・ろ in the following table is 語尾 (common to all Group 2 verbs), and not stem when you talk about 段. In this context 切れ is the only part called stem, hence the name ichidan.

	五段	下一段
未然形	ら・ろ	れ
連用形	り1	れ1
終止形	る	れる
連体形	る	れる
仮定形	れ	れれ
命令形	れ	れろ

---

Kind of related. I think there are vagueness/inconsistencies in terminology.

• Conflicting Definitions of Verb “Roots,” “Bases,” and “Stems”