history - How did "little tsu" become a lengthener?

How did it come about historically that っ preceding a sound would geminate it? Is it really a little つ or are they just near homomorphs?

Answer

This question should be broken into two different questions:

1. When and how did small-tsu come to represent consonant gemination.


2. When and how did consonant gemination (as represented by small-tsu) come to be in Japanese.

(For those who don't know the term: gemination simply means doubling of sounds, usually consonants. It's easy to get the sense once you see it comes from the same Latin root as Gemini).

The answer for the first question is simpler, and was already given: in historical kana usage, consonant gemination was originally represented either by a full-size つ or by another consonant that got elided (usually that would be く, as in 学校, which was written ガくカウ in old spelling, or き as in 石鹸, originally:　セキケン). The small-tsu spelling is recent. It was used sometimes even in the 19th century to indicate that the sound got elided, if not earlier, but small kana letters became a standard only following the 1946 spelling reform.

The answer for the second question is trickier. I don't have any historical linguistics books that I can check to get a definite answer for when the change happened, but it couldn't be earlier than the Heian period, and it was probably earlier than the 17th century. The reason for the change was quite simple: it happened in places where the sounds く and つ were already pronounced without the vowel (as still happens today in words such a ドクター and いつか which are usually pronounced [doktaa] and [itska]).

From here, the process that happened is quite simple: in words like 学校 the gemination was already there once the vowel disappeared and the word was pronounced [gakkou]. In words like 血気, [ketuki] (you have to remember that つ was originally pronounced [tu]) became [ketki] and the /t/ became assimilated to /k/, so in the end we got [kekki].

There are also some cases where the gemination was not caused by an elided consonant, but from the elision of a long vowel (like 真赤　まあか which became 真っ赤　まっか) or just because speakers decided to make the word longer for emphasis or some other reason (that's what happened in きっと, which was originally きと). These spontaneous geminations still happen today.