Long Vowels

In Japanese, long vowels, or "long sounds," chouon 長音, refer to vowels pronounced for twice as long than normal. See also: mora. The opposite are short vowels, or "short sounds," tan'on 短音.

For example, ko こ has a short vowel, while kou こう, koo こー, and koo こぉ have long vowels.

The long vowels are also romanized with a macron, e.g.: Toukyou 東京 becoming Tōkyō, or in other ways, like without macron (Tokyo), or Yuu-gi-ou 遊戯王 becoming Yu-gi-oh.

Orthography

The way long vowels are represented in Japanese is a bit complicated.

In katakana words, the ー prolonged sound mark, also called chouonpu 長音符, "long sound mark," is used.

koohii
コーヒー
Coffee.
keeki
ケーキ
Cake.

Otherwise, a, i, u, e, あいうえ are used.

okaasan
おかーさん
おかあさん
お母さん
Mother.
oniisan
おにーさん
おにいさん
お兄さん
Brother.
fuufu
ふーふ
ふうふ
夫婦
Husband and wife.
oneesan
おねーさん
おねえさん
お姉さん
Sister.

Non-standard pronunciation of long vowels is sometimes rendered with small kana. For example, with contractions:

warii
ワリィ
Bad.
[My] bad. I'm sorry.
- From warui 悪い.
sou ka nou
そうかのぅ
Is that so?
- Dragging the ~o vowel of a the no の sentence ending particle is a way of speaking used by elder characters.
sou itteru yoo
そう言ってるよォ
That's what [I] am saying.
- Here, the yo よ particle was unusually long.

長音化

Some syllables in Japanese are spelled as diphthongs, composed of two different vowels, ~ei and ~ou, but are pronounced in practice as just one long sound, ~ee and ~oo. For example:

sensei
せんせい
せんせー
先生
Teacher.
otousan
おとうさん
おとーさん
お父さん
Father.

Even though in the dictionary the words have ~ei and ~ou, in practice people generally don't pronounce the ~i and ~u separately, they just make the ~e and ~o sound longer than normal.

This process is called chouonka 長音化, literally "long sound-fication."

In such cases, deliberately spelling the word with a prolonged sound mark implies that the character speaks noticeably different from normal. In a very relaxed, or drunk way, or speaking like a little child, that kind of stuff.

短母音化

In some cases, a long vowel is pronounced instead as a short vowel. A phenomenon termed tanboinka 短母音化, "short vowel-fication." This means that a long syllable, which should take two mora to pronounce, takes instead only one mora to pronounce.

Since this change in pronunciation may be impossible to perceive in words spelled with kanji, when it happens the words may be unusually spelled with hiragana, or maybe even spelled with katakana, instead.

For example: