Why was つ originally used to mark consonant gemination? When was that?


I can't find the answer anywhere.

It seems that in manyougana consonant gemination was not marked in any way.
And before the 1946 reform the normal size つ was used instead of the current
smaller one. But what was the original reason to start using つ as the
consonant doubling symbol, and not say く or some original symbol altogether?




**Short summary:** Heian-period Japanese acquired new sounds, including a
syllable-final _-t_ , a geminated _tt_ , and other geminated consonants. The つ
kana, originally _tu_ , was a natural match to write the new _-t_ , which led
to its use in _tt_ , and from that it slowly became the symbol for gemination
in general.

**Long answer:**

Nara-period Old Japanese had no long consonants. This is why the writing
system originally had no way of representing them, and was forced to coöpt
some other symbol into the role. (They _could_ have created a new symbol—it
seems that katakana ン was invented around the 11th century specifically to
write the moraic nasal unambiguously—but people generally seem to prefer
working with the symbols they already know; consider how English writing
failed to create letters to denote all its new vowels, but just make do
awkwardly with the five Latin basic vowel letters.)

In the transition to Heian-period Early Middle Japanese, some very significant
sound changes occurred (the _onbin_ changes). They added, among other things,
long vowels and consonants, and are the source of current verb forms like
_motte_ < _motite_. (For greater clarity, I will use _kunrei rōmaji_ in this
answer; つ = _tu_ , ち = _ti_. Notice these sounds weren't originally pronounced
as in Modern Japanese, but likely as in _ta_.)

Now that Japanese had become a moraic language, they had the problem of how to
represent those new sounds. They might just not represent them at all (e.g.
_motte_ written as モテ; 9th century). Or they might use a number of different
old _kana_ —I don't have a listing of which ones, exactly, but there was
overlap between them and also the new moraic nasal (now written ん); e.g.
gemination could be written as 牟／ム, the usual symbol for the nasal (as if, in
modern _kana_ , you could write _motte_ as もんて).

Around the same period, a second, distinct source of new sounds were Chinese
loanwords—the ancestor of current _kango_ ( _on'yomi_ )—and a lot of those
ended with a syllable-final _-t_ (as in e.g. 罰, modern Japanese _batu_ ,
modern Cantonese _fat_ , Hakka _fat_ ; 達, J. _tatu_ , Cantonese _daat/taat_ ;
日, J. _niti/jitu_ , Cantonese _jat_ , Hakka _ngit_ , Hokkien _jit/lit/git_ ,
and so on). That these sounds were pronounced with a final _-t_ consonant in
Middle Japanese is clear from the descriptions left by the Jesuits in the 16th
century, and also supported by reconstruction.

How was syllable-final _-t_ represented in writing? As you might have guessed,
by the _kana_ symbols for _-tu_ (or, sometimes, _-ti_ ). Recall that in
historical times there were several _kana_ symbols for each sound (the
variants now called _hentaigana_ ). There's evidence that some texts
consistently use some graphical variants for _-tu_ and others for _-t_ ; but
most texts just used an underspecified _-tu_ , and you had to guess which
sound was meant.

Since the _tu_ symbols could denote a single _-t_ , they were a natural choice
for the first _t_ in _tt_ ; _mo-t-te_ = も.つ.て. By extension, つ came to be the
generic symbol of gemination for other consonants (cf. the case of ム above).
All this dates from Heian, but was quite in a state of flux for a long time,
and consolidated slowly. There's probably no sharp line for when did all
geminates could be said to always and consistently be marked by つ; both
sources for this answer only say it "spread gradually" and was settled
"relatively late". The syllable-final _-t_ , too, only became _-ti_ and _-tu_
quite recently (historically speaking).

Sources:

  * Frellesvig, _A History of the Japanese Language_
  * 『日本国語大辞典』, apud [Japanese wikipedia](https://ja.wikipedia.org/wiki/%E3%81%A3).




**Short summary:** Heian-period Japanese acquired new sounds, including a
syllable-final _-t_ , a geminated _tt_ , and other geminated consonants. The つ
kana, originally _tu_ , was a natural match to write the new _-t_ , which led
to its use in _tt_ , and from that it slowly became the symbol for gemination
in general.

**Long answer:**

Nara-period Old Japanese had no long consonants. This is why the writing
system originally had no way of representing them, and was forced to coöpt
some other symbol into the role. (They _could_ have created a new symbol—it
seems that katakana ン was invented around the 11th century specifically to
write the moraic nasal unambiguously—but people generally seem to prefer
working with the symbols they already know; consider how English writing
failed to create letters to denote all its new vowels, but just make do
awkwardly with the five Latin basic vowel letters.)

In the transition to Heian-period Early Middle Japanese, some very significant
sound changes occurred (the _onbin_ changes). They added, among other things,
long vowels and consonants, and are the source of current verb forms like
_motte_ < _motite_. (For greater clarity, I will use _kunrei rōmaji_ in this
answer; つ = _tu_ , ち = _ti_. Notice these sounds weren't originally pronounced
as in Modern Japanese, but likely as in _ta_.)

Now that Japanese had become a moraic language, they had the problem of how to
represent those new sounds. They might just not represent them at all (e.g.
_motte_ written as モテ; 9th century). Or they might use a number of different
old _kana_ —I don't have a listing of which ones, exactly, but there was
overlap between them and also the new moraic nasal (now written ん); e.g.
gemination could be written as 牟／ム, the usual symbol for the nasal (as if, in
modern _kana_ , you could write _motte_ as もんて).

Around the same period, a second, distinct source of new sounds were Chinese
loanwords—the ancestor of current _kango_ ( _on'yomi_ )—and a lot of those
ended with a syllable-final _-t_ (as in e.g. 罰, modern Japanese _batu_ ,
modern Cantonese _fat_ , Hakka _fat_ ; 達, J. _tatu_ , Cantonese _daat/taat_ ;
日, J. _niti/jitu_ , Cantonese _jat_ , Hakka _ngit_ , Hokkien _jit/lit/git_ ,
and so on). That these sounds were pronounced with a final _-t_ consonant in
Middle Japanese is clear from the descriptions left by the Jesuits in the 16th
century, and also supported by reconstruction.

How was syllable-final _-t_ represented in writing? As you might have guessed,
by the _kana_ symbols for _-tu_ (or, sometimes, _-ti_ ). Recall that in
historical times there were several _kana_ symbols for each sound (the
variants now called _hentaigana_ ). There's evidence that some texts
consistently use some graphical variants for _-tu_ and others for _-t_ ; but
most texts just used an underspecified _-tu_ , and you had to guess which
sound was meant.

Since the _tu_ symbols could denote a single _-t_ , they were a natural choice
for the first _t_ in _tt_ ; _mo-t-te_ = も.つ.て. By extension, つ came to be the
generic symbol of gemination for other consonants (cf. the case of ム above).
All this dates from Heian, but was quite in a state of flux for a long time,
and consolidated slowly. There's probably no sharp line for when did all
geminates could be said to always and consistently be marked by つ; both
sources for this answer only say it "spread gradually" and was settled
"relatively late". The syllable-final _-t_ , too, only became _-ti_ and _-tu_
quite recently (historically speaking).

Sources:

  * Frellesvig, _A History of the Japanese Language_
  * 『日本国語大辞典』, apud [Japanese wikipedia](https://ja.wikipedia.org/wiki/%E3%81%A3).

