In this sentence

The missile flew 750 kilometers east at a height lower than 100 kilometers.

appearing here https://www3.nhk.or.jp/news/easy/k10013261541000/k10013261541000.html the recording sounds like there is gemination between the end of ひゃく and the beginning of キロメートル.

What is the orthographically correct way of writing "100km"?


or possibly


  • 6
    I would expect it to be the former since the 促音 comes from 100, thus separating the two words, but I'm only guessing.
    – Simon
    Oct 3, 2021 at 7:55

2 Answers 2


I agree with @Simon in the comments: since the gemination comes from the く in ひゃく, ひゃっ should be the natural choice when writing ひゃっキロ in kana and I would not be too surprised to see

[100]{ひゃっ} [km]{キロメートル}

as furigana in a textbook. (That said, in all textbooks for beginners I could find, there actually seem to be no furigana at all — try to search the web for images with something like 単位 教科書.)

Note that numbers are usually not written in kana, so one should not expect any reliable corpus data to confirm this — e.g. the Balanced Corpus of Contemporary Written Japanese has 0 hits for both ひゃっキロ and ひゃッキロ.

However, @a20 has found a paper titled "Morphology of numerical expressions in contemporary Japanese", where the pronunciation of numbers plus counters in written in kana and you can find the following:

enter image description here

Also interesting is the following:

enter image description here

which says that for 6 and 100 (which end in く), gemination essentially always occurs, but for 1 and 8 (which end in ち), there is some variation.

  • I think ひゃっキロメートル is a perfectly correct way to write this in kana in standard orthography (as long as there is a reason to write this in kana, of course).
    – naruto
    Oct 4, 2021 at 2:07
  • 1
    Logically that makes sense. Googling also agrees, I found one result when using the Katakana version, but a lot when using the Hiragana version. I even found some article from Nara University which had this particular word as an example, among other interesting things: 現代日本語の数詞と助数詞 ー形態の整理と実態調査 repo.nara-u.ac.jp/modules/xoonips/download.php/…
    – a20
    Oct 4, 2021 at 8:40

In response to the comment above...

since the gemination comes from the く in ひゃく, ひゃっ is more sensible than ひゃッ

I can add the following sentence found here



I was relieved when the long vacation came, and when it was nearing the end, it was hard to think that school would start again.

in which we see the following vocabulary item with two different gemination orthographies, according to Jim Breen's wwwjdic:

ホッとした from : ほっと(P); ホッと(P); ホっと (adv,vs) (1) (on-mim) with a feeling of relief; with a sigh of relief; (adv) (2) (on-mim) deeply (sigh)

In light of this example, it consequently appears hard to justify the statement that "gemination comes from" a word, or that any particular orthography is "more sensible." Perhaps responses should be based on objectifiable information, such as frequency of a word's or a spelling's occurrence or at least percentages of their acceptability as determined by polls.

  • 3
    In the example ッ belongs to the onomatopoeic part ホッ and と is written in hiragana, because と is used in its grammatical function of indicating a to-adverb (see this question). The Balanced Corpus of Contemporary Written Japanese has 501 hits for ほっとした and 260 hits for ホッとした, but 0 hits for ホっとした, so there is no ambiguity whether っ belongs to ほ or to と.
    – Earthliŋ
    Oct 10, 2021 at 17:39
  • 3
    Since ひゃっキロ is not really standard orthography (numbers are usually written either as numerals 100 or in kanji), spelling frequency will not be able to help here — e.g. in the Balanced Corpus of Contemporary Written Japanese has 0 hits for both ひゃっキロ and ひゃッキロ, but I'm sure any poll will give the latter close to 0%.
    – Earthliŋ
    Oct 10, 2021 at 17:40

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