Rendaku Explained

What rendaku is

Japanese builds many words by joining two shorter ones. Often the second word keeps its sound: 山 (やま, mountain) plus 道 (みち, road) is 山道 (やまみち, mountain path). But very often the first consonant of the second word changes from voiceless to voiced. That change is rendaku, from 連 (series, joined) and 濁 (muddy, voiced).

• て (hand) + かみ (paper) = 手紙 (てがみ, letter)
• おり (folding) + かみ (paper) = 折り紙 (おりがみ, origami)
• いけ (arranging) + はな (flower) = 生け花 (いけばな, flower arranging)
• ほん (book) + たな (shelf) = 本棚 (ほんだな, bookshelf)
• あお (blue) + そら (sky) = 青空 (あおぞら, blue sky)

The kanji do not change. 花 is the same character in 花 (はな) and 生け花 (いけばな), and 紙 is the same in 紙 and 手紙. Only the reading of the second part shifts, which is why rendaku catches learners who read compounds one kanji at a time.

Rendaku mostly shows up in words made from native Japanese parts, the kunyomi side of the language. It is a real part of how Japanese is spoken, not a spelling rule, so native speakers apply it without thinking and notice at once when a learner says てかみ instead of てがみ. Dictionaries list each compound with its actual reading, voiced or not, so the dictionary entry for the whole word is always the place to check. Rendaku also works on longer words, as in 座布団 (ざぶとん, floor cushion), where ふとん becomes ぶとん.

Which sounds change

Rendaku affects the four rows of hiragana that can take a dakuten, the two small marks that show a voiced sound. The hiragana chart shows each pair side by side. The dakuten and handakuten guide explains the marks themselves.

Notice that the は row becomes the ば row, not the ぱ row. Words starting with な, ま, や, ら or わ never change, because those sounds have no voiced partner. Rendaku is also one of the few places where modern spelling uses ぢ and づ instead of じ and ず: 鼻血 is はなぢ (nosebleed) because the word is はな plus ち (blood), and 三日月 is みかづき because it ends in つき (moon).

Repeated words like 人々 and 時々

Rendaku is especially common when a word is doubled. The mark 々 repeats the previous kanji, and the repeated part usually takes a dakuten:

• 人 (ひと) becomes 人々 (ひとびと, people)
• 時 (とき) becomes 時々 (ときどき, sometimes)
• 山 (やま) becomes 山々 (やまやま, mountains), with no change because ま cannot be voiced
• 木 (き) becomes 木々 (きぎ, trees)
• 国 (くに) becomes 国々 (くにぐに, countries)

Sound-symbolic words are the exception. Mimetic words such as きらきら (sparkling), ころころ (rolling) and ぺらぺら (fluent) repeat without any voicing, so 人々 and きらきら follow different patterns even though both repeat a sound.

When rendaku does not happen

Linguists have found several conditions that usually block rendaku. None of them is perfect, but together they explain most cases.

1. The second part already has a voiced sound. This is Lyman's law, named after Benjamin Smith Lyman, who described it in 1894. かぜ (wind) contains ぜ, so 春風 is はるかぜ, not はるがぜ. In the same way 一人旅 is ひとりたび (traveling alone), not ひとりだび, because たび already contains び. The rule only looks at the second part: a voiced sound in the first part does not block it, as in 袖口 (そでぐち, cuff).
2. The word is Chinese-derived. Compounds built from onyomi, the readings described in onyomi vs kunyomi, usually keep their sounds: 電車 (でんしゃ, train) and 日本 (にほん, Japan) do not change. A minority do, including 株式会社 (かぶしきがいしゃ, corporation) and 親会社 (おやがいしゃ, parent company).
3. The second part is a modern loanword. Katakana words borrowed recently almost never change. A glass case is ガラスケース, not ガラスゲース.
4. The two parts mean X and Y. When the compound lists two things side by side, rendaku is avoided. 山川 read やまかわ means mountains and rivers, while やまがわ means a mountain river. 好き嫌い (すききらい, likes and dislikes) keeps its き for the same reason.

Number sound changes such as 三本 (さんぼん) or 一本 (いっぽん) look similar, but they follow their own patterns after numbers. The Japanese counters guide covers them separately.

Why you cannot always predict it

Even when none of the blocking conditions apply, many compounds simply do not take rendaku. 黒髪 (black hair) is くろかみ, even though 髪 (かみ) is a native word with no voiced sound, while 雨雲 (rain cloud) is あまぐも with the expected change. Some compounds vary between speakers or have two accepted readings, and some, like 山川, change meaning with the reading.

Family names show how unpredictable it is. 島田 is しまだ but 柴田 is しばた, where the voiced ば in the first part seems to discourage voicing in the second. Readers of Japanese names simply learn each name. Because the kanji spelling hides the change, you cannot see rendaku on the page at all, which is also why reading aids like furigana are useful for unfamiliar compounds.

How to learn rendaku

1. Learn compounds as whole words. Study 手紙 = てがみ, not 手 + 紙. This is the same habit that helps with okurigana and jukujikun.
2. Use the patterns as a guess, then check. Expect a dakuten in native compounds, expect none in onyomi compounds or when the second part already has a voiced sound, and confirm in a dictionary.
3. Listen for it. Rendaku is a spoken change first. Hearing a word like あおぞら or ほんだな a few times fixes the voiced sound in memory better than reading it.
4. Watch the kana when you type. A Japanese keyboard will only convert てがみ to 手紙, not てかみ, so typing compounds tests whether you remember the voiced form.
5. Review the ones you get wrong. Add compounds you misread to a spaced repetition deck. Apps such as Moji, which shows example words with readings on each kanji, or Anki can bring them back before you forget.

Rendaku looks like a complication, but it follows a recognizable pattern most of the time. Once you expect the second part of a native compound to soften, words like 人々, 手紙 and 青空 stop surprising you.