Why are the verb classes called ichidan and godan?

Is there a particular reason why verbs are classified as "class 1" verbs (一段動詞) and "class 5" verbs (五段動詞)? Where did class 2 to 4 go? Do or did they exist at all, and why (not)?

Thanks!

Answer

Before considering modern Japanese, I think that it is easier to understand this by first understanding classical. Classical Japanese has three major regular verb classes: quadrigrade (四段), monograde (一段), and bigrade (二段). Both monograde and bigrade may further be sub-divided into upper (上) and lower (下). There are also four irregular classes: k-irregular, s-irregular, n-irregular, and r-irregular. Verbs are conjugated into six forms: irrealis (未然形), adverbial (連用形), conclusive (終止形), attributive (連体形), realis (已然形), and imperative (命令形).

Let's consider the quadrigrade verb kak- 'to write' (書く). When conjugated to each of the above forms, it becomes kak-a, kak-i, kak-u, kak-u, kak-e, kak-e. Notice that the distinctive suffixes are a, i, u, e. The total number of distinctive suffixes is four, which is why this is called quadrigrade (四段).

Next, let's consider the monograde verb mi- 'to see' (見る). When conjugated to each of the above forms, it becomes m-i, m-i, m-iru, m-iru, m-ire, m-iro. Notice how all of the forms contain a single -i. This is why it is called monograde (一段). Now let's consider the verb tabe- 'to eat' (食べる). It conjugates as tab-e, tab-e, tab-eru, tab-eru, tab-ere, tab-ero. This time all of the forms contain a single -e. This too is monograde. To distinguish between these two, those ending in -i are called upper monograde and those ending in -e are called lower monograde. The normal vowel ordering in Japanese is a, i, u, e, o. Notice that comparing i and e, i comes first and e comes second. Traditionally these would be written vertically. As a result, i is considered to be "upper" and e is "lower" in this chart, hence the names.

Next let's consider the bigrade verb ok- 'to rise' (起く). (Older form of modern oki- 起きる 'id'.) When conjugated to each of the above forms, it becomes ok-i, ok-i, ok-u, ok-uru, ok-ure, ok-i[yo]. All forms have an -i or -u in it. This is a two-way distinction, hence the name bigrade. Now let's consider the (older) verb tab- 'to eat' (食ぶ). This conjugates as tab-e, tab-e, tab-u, tab-uru, tab-ure, tab-e[yo]. Again there is a two-way distinction, so this too is bigrade. However, this time the distinction is between -u and -e. As such, bigrade may be sub-classified into upper bigrade (i/u) and lower bigrade (u/e).

The four irregular verb classes are simply irregular which explains their names.

Now let's look at modern Japanese. Modern Japanese simplifies much of the above: all quadrigrade verbs have now become quintigrade (五段), bigrade verbs have gone bye-bye (pun intended), and two of the four irregular classes have disappeared. Also note that the conjugation class formerly known as realis (已然形) is now known as conditional (仮定形).

Let's consider the new quintigrade verb. In addition to the four distinctions earlier, there is now a fifth, hence the name. This fifth form is -o. This can be seen in kak-o: "(I) shall write, let's write". However, this form is secondary in nature: it derives from kak-au (and earlier kak-amu). -au- regularly changes into o: (long o). This -a is the same -a as seen in the irrelias (未然形). This is not a true form, but is added to by-pass the phonological explanation.