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姉 is pronounced _ane_ , but お姉さん is pronounced _oneesan_. What happened to the
initial _a_?

Likewise, 兄 is pronounced _ani_ , but お兄さん is pronounced _oniisan_.

What is the rule governing the change in the pronunciation for the same kanji
in these compound words? Or are these just exceptions one simply needs to
remember?
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words pronunciation readings
Why are 兄(あに) and 姉(あね) pronounced differently in お兄さん and お姉さん?
# There is no simple rule

First of all, let us take a look whether variant pronunciations exist.

There are some instances of 兄様【あにさま】, 兄【あに】さん, 姉様【あねさま】, 姉【あね】さん found in
several writings accessible on the free digital library 青空文庫, all from authors
around 1900.

> **おみの** （あたりを窺ひて。）子之介【ねのすけ】は厩にゐると御門で教へられたが、はて何處へ行つたことであらう。
>
> （奧より子之介出づ。）
>
> **おみの** おゝ、弟……。
>
> **子之介** 姉樣【あねさま】か。（なつかしげに寄る。）ようたづねて來てくだされた。
>
> ――岡本綺堂【おかもときどう】(1914) 『佐々木高綱【さゝきたかつな】』

Also, in the movie もののけ姫, _Kaya_ apparently calls _Ashitaka_ , who is not her
brother, あにさま, as an expression of her affection or reverence.

In fact, 姉様 is even listed in some dictionaries:

> あね‐さま【姉様】
>
> ① 姉を敬っていう語。
>
> ② 若い女性を親しんでいう語。
>
> ③「姉様人形」の略。
>
> ――大辞林

The same dictionaries lists 姉【あね】さん and 兄さんas well, including sense ①, ie.
`（姉・兄） 姉を親しみ敬っていう語`.

Note that there are also some compound words where あに and あね do not get
shortened:

> 姉上【あねうえ】, 兄上, 姉貴【あねき】, 兄貴

Today the versions without the initial mora are more common. Considering that
the longer versions did exist, it is simply a shortening of the compound
words, perhaps because おねえさん sounds smoother or is more easily pronounced, or
perhaps to parallel お母【かあ】さん, お父【とう】さん, おば(あ)さん, おじ(い)さん.

There do exists some regular phonetic changes such as あう→おう, but they don't
seem to apply here.

To put it in a perspective, shortening words in compounds is not a new
phenomenon:

  * 風の音【と】
  * 淡海【あふみ】 (←淡【あは】海【うみ】)
  * かわら (←河原【かわはら】)
  * 盥【たらい】 (←手洗い)
  * はとり(←機織り【はたおり】)
  * 鍛冶【かぬち】 (←金打ち【かねうち】)
  * 八咫【やた】 (←八【や】-咫【あた】)

And it's still being done:

  * アー写 (←アーティスト写真)
  * キー坊【ぼう】 (←キーア)
  * ゆーたん (eg. ←ゆきの)
  * テレくん (←テレフンケン)
  * パネェ (←半端ない)
  * ラノベ (←ライトノベル)
  * テレコ (←カセット **テ** ープ **レコ** ーダー)
  * ネカフェ (←インターネットカフェ)
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