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I am a beginner in Japanese and I've noticed that when two words are
compounded into one word the first letter of the second word changes.

hira + kana = hiragana

ko + hayashi = kobayashi

ori + kami = origami

This pattern would imply that the word Katakana should be Katagana.

Any ideas as to why this is ?

NB. I am not referring to Kanji having different readings in Japanese. That is
a different issue. I am specifically referring to compound words where the
first letter of the second word undergoes a change.
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2019-07-16T10:45:09.380
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grammar pronunciation compounds rendaku
Why is Katakana not pronounced Katagana?
Unfortunately I think there is no logical explanation.

Even a pair of 2 identical kanjis may have "both versions" eg when used in
names.

One example is 大島 ("big island"), with the "大島" that island being southwest of
Tokyo, close to Izu peninsula is "shima", there is a railway station in Tokyo
with the same kanji, but that one is "jima"
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This phenomenon is called **連濁** (rendaku). The basic rules for rendaku can be
found in the following question, so please take a look at it first:

  * [Rules or criteria for 連濁: Voiced or unvoiced syllables in compound words](https://japanese.stackexchange.com/q/2526/5010)

Now, in addition to the rules mentioned in the linked question, there is yet
another rule (or "tendency") regarding rendaku: **there are several kanji that
tend to block rendaku for whatever reasons**.

According to Mark Irwin, a prefix 片- (かた; meaning "part", "fragment") is one
of such kanji, and many words that start with 片 somehow block rendaku. See
[this presentation
(PDF)](http://www-h.yamagata-u.ac.jp/~irwin/site/Home_files/Rendaku%20Dampening%20%28web%20handout%29.pdf)
for details.

  * 片言 かたこと
  * 片恋 かたこい
  * 片時 かたとき
  * 片仮名 かたかな

* * *

Likewise, 御【お】-, 御【ご】-, 一【ひと】-, 二【ふた】-, 唐【から】- and so on tend not to accept
rendaku:

  * 御酒 おさけ
  * 一葉 ひとは
  * 唐傘 からかさ

Some kanji including -姫【ひめ】, -先【さき】, -浜【はま】 block rendaku when it's the second
component of a compound:

  * 砂浜 すなはま
  * 歌姫 うたひめ

Unfortunately, even researchers do not know why these kanji block rendaku. It
may be worth remembering which kanji tends to block rendaku, but please keep
in mind that there are many exceptions like 二葉（ふたば）. Ultimately, you have to
remember each word individually.
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2019-07-16T13:30:46.063
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