Spelling Long Vowels: おう vs おお, えい vs ええ

You hear a long o in a word and reach for the kana — but do you write it おう or おお? You hear a long e — is it えい or ええ? The two spellings sound identical, so your ear cannot decide for you. This page gives the rule that settles almost every case, then the short, finite list of native words that break it, and — crucially — why they break it, so the exceptions stop feeling random. If you want the sound side of long vowels first, start with long vowels in hiragana; this page is the orthographic deep dive.

The default: おう and えい

For the overwhelming majority of words, the rule is simple:

• A long o is written おう — お followed by う.
• A long e is written えい — え followed by い.

The second kana does not add a separate u or i sound; it is a length marker. がっこう is gakkō, not "gakko-u"; せんせい is sensē, not "sen-say-ee."

おはよう (ohayō), 今日 (きょう kyō), そう (sō) all take おう; 先生 (せんせい sensei, pronounced sensē) takes えい. Sino-Japanese readings almost always follow the default, so とうきょう (東京 Tōkyō), がっこう (学校 gakkō), and えいが (映画 eiga, "movie") are all regular.

The exceptions: a finite おお list

A small, closed set of native words writes a long o as おお — doubled お — instead of おう. There is no sound difference (おおきい and a hypothetical おうきい would be pronounced the same), so these cannot be derived; they must be memorized. The good news is that the list is short and the words are common.

Beyond this everyday core there are a few rarer おお words — おおやけ (公, ōyake, "public"), もよおす (催す, moyōsu, "to hold an event"), とどこおる (滞る, todokōru, "to stagnate"), いきどおる (憤る, ikidōru, "to be indignant") — but the twelve in the table cover almost everything a learner meets.

Why these words, and not others?

The exception list looks arbitrary until you learn its origin — and then it becomes almost predictable. The おお words are historical relics. In older Japanese, each of them had a は-row kana (は, ひ, ふ, へ, ほ) sitting between the two o-vowels. Over centuries that medial consonant softened to a w sound and then dropped out entirely, leaving two adjacent o's that fused into a long ō — and the spelling froze with two お.

By contrast, the default おう words got their length from a genuine historical u: either a Sino-Japanese reading that really ended in -u (校 was kau/kou → kō), or the adjective sound-change く → う that created arigataku → ありがとう (arigatō) and hayaku → おはよう (ohayō). In short: おう comes from a real u; おお comes from a collapsed は-row syllable. That is why the おお set is finite — it is exactly the words that once had a ほ (or は/ひ/ふ/へ) in the middle — and why it is worth memorizing as a list rather than trying to hear it.

The long e exceptions: ええ

Long e is even more lopsided. The default えい covers nearly everything (せんせい, えいが, けいざい keizai "economy", とけい tokei "clock"). Only a tiny native set writes long e as ええ:

おねえさん is the one that catches learners, because its "family" relatives are regular: おにいさん (お兄さん, older brother) doubles い to いい, and おとうさん (お父さん, father) takes the おう default. Only おねえさん breaks ranks with ええ. The interjections (ええ, ねえ, へえ) are (informal) and often stretched further in casual writing as えー, ねー — with the katakana-style bar even in hiragana speech.

The whole thing is hiragana-only

None of this applies in katakana. Loanwords write every long vowel — o, e, a, i, u alike — with the single long-vowel bar ー, so the おう/おお and えい/ええ puzzle simply does not exist there.

ケーキ (kēki, "cake") and コーヒー (kōhī, "coffee") show a long e and long o both written with ー, no choice required. So the entire おお/ええ headache is a native-word, hiragana-only phenomenon. For the bar itself, see the katakana chōonpu ー.

Minimal pairs: the spelling really distinguishes words

Because the two long-o spellings can front different words, choosing wrong is not just a typo — it can point at a different word entirely.

氷 must be こおり; write こうり and you have named 小売 ("retail") instead. Likewise 大 words are おお (おおきい, おおい) while 王 "king" words are おう (おうさま ōsama, "king"; おうじ ōji, "prince"), even though both readings are ō.

Common mistakes

The two errors that dominate are over-applying the default (writing おおきい as おうきい because "long o is usually おう") and reading えい as two separate vowels ("sen-say-ee") instead of a single held ē. The first is cured by memorizing the twelve-word おお list; the second by remembering that the second kana in おう/えい is a length marker, not a new sound.

Key takeaways

• Default spellings: long o → おう, long e → えい. The second kana marks length, not a new vowel.
• A finite native list breaks the o rule with おお: おおきい, おおい, とおい, とお, とおる, とおす, こおり, こおる, おおう, おおかみ, ほのお, とおか.
• The tiny ええ set: おねえさん, ええ, ねえ, へえ.
• These おお/ええ words are historical relics (a lost は-row syllable), which is why the list is closed and worth memorizing.
• The whole issue is hiragana-only; katakana lengthens every vowel with the bar ー (ケーキ, コーヒー).