What Is Rendaku? Check the Sound Change in a Japanese Compound
A compound can sound different from its separate parts. Follow the change from medicine to eye medicine, locate the consonant that changes, and keep the checked whole-word reading instead of applying voicing everywhere.
Inku editorial team · Published · 3 min read
When words combine, the first consonant of the second part can change, such as k becoming g. This change is called 連濁 (れんだく, rendaku, "sequential voicing"). The change happens in some compounds, so it is a reason to check a whole word's reading, rather than a rule to apply everywhere.
One checked compound
A compound is a word formed by combining parts that contribute to its meaning. The TUFS sequential-voicing explanation uses these parts:
- 目 (め, me, "eye") supplies the first part.
- 薬 (くすり, kusuri, "medicine") supplies the second part.
- 目薬 (めぐすり, megusuri, "eye drops") is the complete compound.
The second part begins with a k sound when it stands alone. In this compound, that starting consonant changes to g. The first part keeps its reading. Locate that change before trying to memorize the complete word.
You can see the difference in kana: く (く, ku, "the ku sound") becomes ぐ (ぐ, gu, "the gu sound") at the start of the second part. The kana mark makes the change visible. The two kanji in the compound do not receive that mark.
Find the right part
A useful explanation identifies both the position and the change. Saying that a compound “changes pronunciation” is too vague if you cannot point to the part affected.
- Read the medicine word on its own using the checked reading above.
- Read the complete eye-drops word.
- Compare the beginning of the second part: k in the separate word, g in the compound.
- Return to the complete compound and its meaning.
The last step matters. The component comparison explains something about the word you are learning; it does not replace learning that word. Keep the full spelling, reading and meaning together.
Keep the rule conditional
TUFS describes cases where the change happens, rather than saying every compound must undergo it. A beginner should not change every second element just because two written parts appear together.
If a reading differs from your guess, check a dictionary entry or course example for that exact word. Do not “correct” the entry to match a pattern you expected. A confirmed reading is stronger evidence than a guessed transformation.
Keep uncertain names and unfamiliar compounds in the same category: readings to verify. This short explanation does not provide a complete prediction system or an accuracy percentage. It gives you a way to notice the relevant difference and ask a precise question.
Separate three reading questions
The on'yomi and kun'yomi guide explains the origins and categories of kanji readings. Rendaku asks a different question: does the starting consonant change when this part joins the compound?
The guide to dakuten and handakuten explains the written kana marks. Recognizing a mark helps you read the checked kana form. It does not tell you in advance which unfamiliar compounds will use that form.
Finally, remembering a complete word is a vocabulary task. Use the vocabulary-learning guide if you recognize an explanation but forget the word later.
Keep one whole reading
Write one small note containing the compound, its checked reading and meaning. Add a component comparison only when it helps explain the actual reading. That keeps the observation tied to evidence.
For your next step, browse Inku's public vocabulary library for a familiar compound. Check its whole-word reading against a reliable reference before deciding whether the same sound change occurs.
Sources
- TUFS: sequential voicing in a medicine compound — Conditional voicing at the beginning of a compound's second element; the eye, medicine and eye-lotion example supplies the written forms, readings and meanings. (checked 2026-10-17)
Prepared by our editorial team with AI assistance. Product details are checked against the sources below.