For example:

  • て+つな=たづな(手綱)
  • め+ふた=まぶた(瞼・目蓋)
  • かね+つち=かなづち(金槌)

The only thing I can see for sure is that the second word becomes voiced, but that's more of an after-the-fact thing than a rule that dictates when the sound actually changes from e to a.

Is there a rule or pattern to it?


1 Answer 1


e does not become a. Rather, it is the other way around: a becomes e. More specifically, there are two forms of e: e1 and e2. (See 上代特殊仮名遣) The rule is a + i > e2. Both e1 and e2 merge into e after Nara period.

For reference, these pairs are termed 被覆形 and 露出形. The form without the -i suffix is 被覆形 while the -i suffixed form is 露出形.

  • 1
    Here's a list of examples.
    – jogloran
    Commented Nov 6, 2012 at 5:49
  • 1
    You mention that a+i > e2 > Modern e, and that the original root ends in a. What is the i which yields modern e when the word occurs free and not in a compound?
    – jogloran
    Commented Nov 6, 2012 at 5:50
  • That explains the surname 金井 and others, too. For some reason I didn't even connect this in my mind to other similar transformations with other sounds. Thank you!
    – ssb
    Commented Nov 6, 2012 at 6:13
  • @jogloran It is sometimes said to be some kind of subject or topic marker.
    – Zhen Lin
    Commented Nov 6, 2012 at 23:43
  • @ZhenLin The subject / topic marker -i is usually considered to be separate. Also, due to the skewed distribution, it is argued to be loan from Old Korean. There are some fossilized forms surviving such as arui (wa). The -i mentioned here with nominals is more likely to be *C > *y > i, where C is some consonant that lenited.
    – Dono
    Commented Nov 7, 2012 at 0:11

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