What are the differences between じ and ぢ, and ず and づ?


The Japanese hiragana and katakana syllabaries can mostly be described as
phonetic. But there are two exceptions, the two pairs of syllables modified to
be voiced with the _dakuten_ diacritic which turns them into homophones:

  * す (su) → ず (zu); つ (tsu) → づ (zu) 
  * し (shi) → じ (ji); ち (chi) → ぢ (ji)

The same goes for katakana:

  * ス (su) → ズ (zu); ツ (tsu) → ヅ (zu)
  * シ (shi) → ジ (ji); チ (chi) → ヂ (zi)

So are these pairs always pronounced the same or are there sometimes subtle
differences? The former of each pair seems to be the more common, are the
latter in each pair much used? How does one choose which to use when spelling
words in kana? What about for spelling foreign words or names in kana?




In modern Japanese these pairs are pronounced exactly the same:

  * ず, づ are pronounced either `[dzu]` or `[zu]`. 
  * じ, ぢ are pronounced either `[dʑi]` or `[ʑi]`.  
(the first sounding like the English J and the second like the French J, but
both are with the middle of the tongue raised to the hard palate, producing
what seems like a softer pronunciation).

So in short, the pairs are redundant as far as the modern language is
concerned. But like most cases of duplicate letters, they originally
represented distinct sounds:

  * ず represented `[zu]` (nowadays it is really more like `[dzu]` with `[zu]` being a slacker pronunciation).
  * づ represented `[du]` (and just like `[tu]` became `[tsu]` it became `[dzu]`)
  * じ represented `[zi]` (and just like `[si]` became `[ɕi]` it became `[ʑi]`).
  * ぢ represented `[di]` (and just like `[ti]` became `[tɕi]` it became `[dʑi]`).

While the exact pronunciation of these 4 letters have changed since classical
Japanese, they essentially remained distinct until the Edo period. We can
easily see this from old Portuguese transliterations of Japanese, which used
the following letters to transliterate these sounds:

  * じ was transliterated `ji`.  
(remember that this is the Portuguese or French `j` here, not the English
`j`).

  * ぢ was transliterated `gi`.  
(it was obviously distinct from ぎ, but not distinct enough to Portuguese or
Spanish ears)

  * ず was transliterated `zu`.
  * づ was transliterated `zzu` or `dzu`  
I'm not sure how this last letter was pronounced, since the only old text I've
read, _Ars Grammaticae Japonica_ , makes a clear distinction between 水 (old
spelling: みづ) which it transliterates as _mizzu_ and 蜜 (strangely enough: みつ)
which it spells as _mizzu_. This mess probably stems from the fact that
Spanish itself (the writer is Spanish) was undergoing transformations to the
very same consonants at that time.

In the end, `[dzu]` merged with `[zu]` and `[dʑi]` merged with `[ʑi]` in all
but a few dialects (the Kagoshima dialect apparently retains these
distinctions), but until the spelling reforms of 1946 all words retained their
original spellings, so 水 was spelling ミヅ in and 味 was spelled アヂ. Good kokugo
(Japanese-Japanese) dictionaries still list these old spellings.

After the spelling reforms, づ and ぢ were kept in only two places:

  1. With a repeated sound: [続く]{つづく}, [縮む]{ちぢむ}
  2. In compounds with [rendaku](http://en.wikipedia.org/wiki/Rendaku): [気付く]{きづく}, [馬鹿力]{ばかぢから}

In all of the other places, ぢ was replaced with じ and づ with ず.

