づ vs ず (and ぢ vs じ): The Japanese Spelling Rule for Yotsugana
づ and ず sound identical. So do ぢ and じ. Yet 水 is written みず and 続く is written つづく — and getting it wrong is a classic mistake (even for natives, and for the IME). These four kana are called yotsugana (四つ仮名), and the rule for choosing between them is refreshingly simple.
The Default: Always ず and じ
In modern Japanese spelling (現代仮名遣い), the “zu” sound is written ず and the “ji” sound is written じ — unless one of two specific exceptions applies. When in doubt, use ず/じ.
Exception 1: Rendaku (連濁)
When a word that begins with つ or ち joins a compound and becomes voiced, it turns into づ or ぢ — keeping its connection to the original word.
| Original word | Compound | Reading |
|---|---|---|
| 月（つき） | 三日月 | みかづき (crescent moon) |
| 遣い（つかい） | 小遣い | こづかい (pocket money) |
| 作り（つくり） | 手作り | てづくり (handmade) |
| 血（ち） | 鼻血 | はなぢ (nosebleed) |
| 力（ちから） | 底力 | そこぢから (latent strength) |
つ → づ, ち → ぢ. The voiced kana “points back” to the word it came from.
Exception 2: Repeated つ / ち
When つ or ち is repeated within a word and the second one voices, it is written づ / ぢ.
つ + つく — repeated つ
ち + ちむ — repeated ち
Words People Get Wrong
repeated つ (Exception 2)
「地」 on-reading is じ — not rendaku
modern standard is ず (いなづま also accepted)
Typing & Checking
On a romaji keyboard: ず = zu, づ = du, じ = ji, ぢ = di.
Let the IME confirm you: type tuduku and 続く appears; tuzuku will not convert to it. If the kanji you expect does not come up, your kana is wrong.
Teacher’s Note
Boil it down to one sentence: default to ず/じ; use づ/ぢ only when the sound comes from a つ/ち (rendaku) or a repeated つ/ち. Watch 地面 (じめん) — the じ is an on-reading, not rendaku, so it is じ, not ぢ.
Spell it right, every time
Nihongo Pass pairs every word with its correct reading and audio — so づ/ず and ぢ/じ stop being a guessing game.
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