The core idea: two words fuse, and the join softens
Te (手) is hand and kami (紙) is paper. Put them together and you get tegami (手紙), a letter, literally hand paper. The k of kami has become g. Hito (人), person, said twice, gives hitobito (人々), people, with a b where the second h was. This is rendaku, and once you know it you will hear it everywhere.
Think of it as glue. Two separate words said one after the other keep their own sounds. When they fuse into a single word, the consonant at the join often softens into its voiced partner, which tells the listener that this is now one word. Only the first consonant of the second part changes; everything else stays as it was.
The diagram shows one compound per sound pair; the sentences after it use the same words at home, at work and among friends.
| Japanese | Romaji | Meaning |
|---|---|---|
| 母に手紙を書きました。 | Haha ni tegami o kakimashita. | I wrote a letter to my mother. |
| 今日は青空がきれいですね。 | Kyō wa aozora ga kirei desu ne. | The blue sky is lovely today, isn't it? |
| その本は本棚に戻してください。 | Sono hon wa hondana ni modoshite kudasai. | Please put that book back on the bookshelf. |
| 駅の前は人々でいっぱいでした。 | Eki no mae wa hitobito de ippai deshita. | It was packed with people outside the station. |
| 週末、花火を見に行きませんか。 | Shūmatsu, hanabi o mi ni ikimasen ka. | Would you like to go and see the fireworks this weekend? |
Which sounds change: four pairs and two spelling surprises
Rendaku only touches consonants with a voiced partner written with dakuten: k, s, t and h, plus their variants sh, ch, ts and f. If the second part starts with a vowel or with n, m, y, r or w, there is nothing to voice, which is why kaimono (買い物), shopping, looks untouched.
The h row is the odd one out, because it becomes b rather than a voiced h. The reason is history. In old Japanese that row was pronounced with a p sound, later an f, and the voiced partner of p is b. The sound moved on to h, but its old partner stayed.
Three words from the table below come up at a clinic, on an evening walk and at a party.
| Japanese | Romaji | Meaning |
|---|---|---|
| 急に鼻血が出ました。 | Kyū ni hanaji ga demashita. | My nose suddenly started bleeding. |
| 今夜は三日月がきれいですね。 | Kon'ya wa mikazuki ga kirei desu ne. | The crescent moon is beautiful tonight. |
| これ、母の手作りのケーキです。 | Kore, haha no tezukuri no kēki desu. | This is a cake my mother made herself. |
| Compound | Reading | Change | Literally |
|---|---|---|---|
| 手紙 | tegami | ka becomes ga | hand paper: a letter |
| 出口 | deguchi | ku becomes gu | go-out mouth: an exit |
| 青空 | aozora | so becomes zo | blue sky |
| 巻き寿司 | makizushi | su becomes zu | rolled sushi |
| 本棚 | hondana | ta becomes da | book shelf: a bookshelf |
| 鼻血 | hanaji | chi becomes ji, spelled with chi | nose blood: a nosebleed |
| 三日月 | mikazuki | tsu becomes zu, spelled with tsu | third-day moon: a crescent moon |
| 花火 | hanabi | hi becomes bi | flower fire: fireworks |
Hanaji and mikazuki solve a spelling puzzle. Modern kana writes ji and zu as じ and ず almost everywhere, but when they come from chi or tsu through rendaku, the original kana simply gains the dots: hanaji (はなぢ), mikazuki (みかづき), tezukuri (てづくり), handmade. The post on dakuten and handakuten covers the marks themselves.
Beyond the basics: what keeps the join hard
So far rendaku looks like a rule. It is not, and the useful question is when it does not happen. A handful of blockers explain most cases, and each follows from the glue idea: something about the second word keeps the parts feeling separate, or makes a second voiced sound unwelcome. Run a compound through the gates from top to bottom.
- Nothing to voice: kaimono, shopping, since mono starts with m.
- A loanword: kamikoppu, paper cup.
- A word of Chinese origin: eikaiwa, English conversation.
- A pair of equals: yamakawa, mountains and rivers.
- Already voiced: kamikaze, divine wind, since kaze contains z.
The strongest blocker: a voiced sound already in the second word
Kaze, wind, contains z. Put it after kami, god, and you get kamikaze, not kamigaze. Kagi, key, contains g, so a spare key is aikagi, literally a matching key. Fuda, tag, contains d, so a name tag is nafuda. Tabi, journey, contains b, so travelling alone is hitoritabi. This is Lyman's law, the most reliable of all the tendencies.
The reason is a sound pattern of native Japanese: a native word very rarely contains two voiced consonants of the g, z, d, b kind, and voicing the start of kaze would create a second one. Only the second word counts. The first part may contain voiced sounds freely: kaze, a cold, plus kusuri, medicine, gives kazegusuri. The best-known exception is nawabashigo, rope ladder, voiced despite the g in hashigo.
These words turn up at a key-cutting counter, a conference, a winter walk, a chat about travel and a pharmacy.
| Japanese | Romaji | Meaning |
|---|---|---|
| 合鍵を一本作りたいんですが。 | Aikagi o ippon tsukuritai n desu ga. | I'd like to have a spare key made. |
| 会議の間は名札をつけてください。 | Kaigi no aida wa nafuda o tsukete kudasai. | Please wear your name tag during the meeting. |
| 今日は北風が冷たいですね。 | Kyō wa kitakaze ga tsumetai desu ne. | The north wind is cold today, isn't it? |
| 去年、一人旅で北海道に行きました。 | Kyonen, hitoritabi de Hokkaidō ni ikimashita. | Last year I travelled to Hokkaido on my own. |
| 風邪薬はありますか。 | Kazegusuri wa arimasu ka. | Do you have any cold medicine? |
Pairs of equals: yamakawa versus yamagawa
Some compounds are not one thing described by another but two things side by side: mountains and rivers, reading and writing, likes and dislikes. The parts are listed rather than fused, so, as the glue idea predicts, the join usually stays hard.
The clearest proof is one compound with two readings. Written 山川, yamakawa means mountains and rivers, while yamagawa is a river in the mountains, literally a mountain river. The kanji are identical, and only the voicing tells you which relationship is meant. In the table, each pair of equals sits beside a fused compound that shares its second word.
A host asking about food, a student describing her Japanese and a poster for a family event all use pairs of equals.
| Japanese | Romaji | Meaning |
|---|---|---|
| 好き嫌いはありますか。 | Sukikirai wa arimasu ka. | Is there anything you don't eat? |
| 読み書きはできますが、会話はまだ苦手です。 | Yomikaki wa dekimasu ga, kaiwa wa mada nigate desu. | I can read and write, but conversation is still hard for me. |
| 親子で参加できます。 | Oyako de sanka dekimasu. | Parents and children can take part together. |
| Compound | Reading | Meaning | Why |
|---|---|---|---|
| 山川 | yamakawa | mountains and rivers | Two equals: no rendaku |
| 山川 | yamagawa | a mountain river | A kind of river: rendaku |
| 読み書き | yomikaki | reading and writing | Two equals: no rendaku |
| 手書き | tegaki | handwritten, literally hand writing | A way of writing: rendaku |
| 好き嫌い | sukikirai | likes and dislikes | Two opposites: no rendaku |
| 人嫌い | hitogirai | someone who avoids people | A kind of dislike: rendaku |
| 親子 | oyako | parent and child | Two equals: no rendaku |
| 双子 | futago | twins, literally paired children | Two children as one unit: rendaku |
Loanwords and Chinese-origin words keep their shape
Rendaku belongs to the native layer of Japanese. Loanwords almost never take it: kamikoppu, paper cup; kan kōhī, canned coffee. Words borrowed from Chinese, read with on readings, mostly resist it too: eikaiwa, English conversation; fukushachō, vice-president. They arrived as sound blocks of their own and kept that feeling.
The exceptions confirm the logic. Words used so often that they feel native have joined the glue: kaisha, company, becomes gaisha in kabushikigaisha, joint-stock company, and ryokōgaisha, travel agency; futon becomes buton in zabuton, floor cushion, literally sitting futon. A few Portuguese words from the sixteenth century do the same: karuta becomes garuta in iroha-garuta. You might hear these at work and at home.
| Japanese | Romaji | Meaning |
|---|---|---|
| 紙コップはどこですか。 | Kamikoppu wa doko desu ka. | Where are the paper cups? |
| 缶コーヒーを二本買ってきました。 | Kan kōhī o ni-hon katte kimashita. | I bought two cans of coffee. |
| 英会話の先生をしています。 | Eikaiwa no sensei o shite imasu. | I teach English conversation. |
| 旅行会社に勤めています。 | Ryokōgaisha ni tsutomete imasu. | I work for a travel agency. |
| どうぞ、座布団をお使いください。 | Dōzo, zabuton o otsukai kudasai. | Please, use this cushion. |
The honest part: words and names you learn one by one
Some words pass every gate and still refuse. Tori, bird, becomes dori in wataridori, migratory bird, but stays tori in yakitori and kotori, little bird. Kutsu, shoe, becomes gutsu in nagagutsu, rubber boots, yet kutsushita, socks, keeps its sh, and himo, string, stays himo in kutsuhimo, shoelace. No gate predicts these; you learn them by hearing them.
Names are the hardest case. The same kanji can be Nakajima or Nakashima, Yamazaki or Yamasaki, and each family keeps its own version. Japanese people check too, which is why business cards often show a reading, and asking in a first meeting is perfectly polite.
| Japanese | Romaji | Meaning |
|---|---|---|
| 焼き鳥を二本ください。 | Yakitori o ni-hon kudasai. | Two skewers of yakitori, please. |
| 靴ひもがほどけていますよ。 | Kutsuhimo ga hodokete imasu yo. | Your shoelace is undone. |
| 失礼ですが、なかじまさんですか、なかしまさんですか。 | Shitsurei desu ga, Nakajima-san desu ka, Nakashima-san desu ka. | Excuse me, is it Nakajima or Nakashima? |
| やまさきと読みます。 | Yamasaki to yomimasu. | It's read Yamasaki. |
Common rendaku mistakes and where they come from
Saying tekami or hontana, with the second word in its dictionary form. The cause is building compounds from parts you already know. The fix is to learn each compound as a whole word with its own reading.
Voicing a loanword: kamigoppu for kamikoppu. The cause is over-applying the pattern once you have noticed it. Katakana is your warning sign: a katakana second half almost never changes.
Assuming the first part matters, and saying kazekusuri because kaze already has a z. Only the second word is checked, so kazegusuri is correct.
Voicing words of Chinese origin: eigaiwa for eikaiwa. Sino-Japanese compounds have their own join change, a doubled consonant, as in gakkō, school, literally study school, from gaku and kō. The post on long vowels and double consonants covers that small tsu; rendaku is the native pattern.
Guessing at names. A misread name is remembered. Check the card or ask, and use that reading from then on.