Korean 사이시옷: Why It's 햇빛 and 나뭇잎 but 개수 and 해님

Korean writes a small ㅅ between the two parts of some compound words, as in 햇빛 (sunlight) and 나뭇잎 (tree leaf), and leaves it out of others, as in 해님 and 개수. The rule is Article 30 of the Hangeul orthography rules, published by the National Institute of Korean Language (NIKL). The ㅅ is called 사이시옷 ("in-between ㅅ"), and it appears only when three conditions are all true.

1. The three conditions

NIKL's commentary on Article 30 sets out the test.

1. The word must be a compound. A compound joins two words. A word made with a suffix does not count. 해 (sun) plus the suffix -님 gives 해님, not 햇님. The compound 해 + 빛 gives 햇빛.
2. A sound change must happen at the join. One of three things must occur:
   • The first sound of the second part becomes a tense sound: 바다 + 가 is read [바다까], so it is written 바닷가.
   • An ㄴ sound is added before ㄴ or ㅁ: 비 + 물 is read [빈물], so it is written 빗물. Likewise 코 + 날 is read [콘날], so it is written 콧날.
   • A double ㄴㄴ sound is added before a vowel: 예사 + 일 is read [예ː산닐], so it is written 예삿일.
3. At least one part must be native Korean, and no part may be a loanword. If both parts are Sino-Korean, the ㅅ is not written.

Article 30 covers only compounds whose first part ends in a vowel.

2. Examples from the rule

Words that join a native part and a Sino-Korean part follow the same logic: 자릿세, 전셋집, 아랫방, 훗날, 제삿날, 가욋일.

3. Why 개수, 초점 and 전세방 have no ㅅ

NIKL's commentary names common mistakes. 개수 (個數), 초점 (焦點), 기차간 (汽車間) and 전세방 (傳貰房) are often miswritten as 갯수, 촛점, 기찻간 and 전셋방. All four are built entirely from Sino-Korean parts, so condition 3 fails and there is no ㅅ.

The same goes for loanword compounds. NIKL gives 오렌지빛 and 피자집 as examples without ㅅ, because 오렌지 and 피자 are loanwords. By contrast, NIKL lists 장밋빛 and 보랏빛 among the words that do take ㅅ.

4. The six Sino-Korean exceptions

Article 30 lists exactly six two-syllable Sino-Korean words that do take 사이시옷:

곳간 (庫間), 셋방 (貰房), 숫자 (數字), 찻간 (車間), 툇간 (退間), 횟수 (回數)

NIKL says that no other Sino-Korean word takes it. That explains the pair 횟수 and 개수: both mean "count," but only 횟수 is on the list. It also explains 외과, 이비인후과 and 국어과, which are written without ㅅ.

5. A trap with 과 (科)

In biological classification, 과 means "family." When a Sino-Korean word precedes it, as in 장미과 (rose family), no ㅅ is written. When a native word precedes it, the compound has a native part, so the ㅅ appears: 고양잇과, 소나뭇과, 가짓과, 멸칫과.

6. 윗 and 위

NIKL's commentary uses 위 (top) to show that the sound test decides. With 길 and 물 the compound is written 윗길 and 윗물. With 턱 and 쪽 it is written 위턱 and 위쪽, because nothing changes at the join.

7. What is not 사이시옷: Article 31

A different pattern is sometimes confused with it. Article 31 says that when a ㅂ or ㅎ sound is added in a compound, you write the sound you hear:

• ㅂ added: 볍씨, 멥쌀, 햅쌀, 좁쌀, 입때
• ㅎ added: 살코기, 수캐, 수컷, 수탉, 암컷, 암탉

NIKL explains that these ㅂ and ㅎ are traces of older forms of the first or second word. It also says not every 수 (male) word takes the ㅎ: 수고양이 is not written 수코양이.

8. A quick check before you write ㅅ

1. Is the word a compound of two words (not word plus suffix such as -님)?
2. Does it include a native Korean word, and no loanword?
3. Does the join make a tense sound, an added ㄴ, or an added ㄴㄴ?
4. If both parts are Sino-Korean, is it one of the six listed words?

If all answers are yes, write 사이시옷.

9. Key takeaways

• 사이시옷 is written only in compounds, never in words with a suffix: 해님, not 햇님.
• The join must change the sound: a tense sound, an added ㄴ, or an added ㄴㄴ.
• At least one part must be native Korean, and no loanword may be in the compound.
• Only six two-syllable Sino-Korean words take it: 곳간, 셋방, 숫자, 찻간, 툇간, 횟수.
• 개수, 초점, 기차간 and 전세방 are all Sino-Korean and have no ㅅ.
• Article 31 is separate: 볍씨, 햅쌀 and 살코기 are spelled as they sound.