Definition
In Standard South Korean, the initial sound rule (Dueum Law) is a phonological and spelling rule where specific consonants are avoided at the beginning of a word, causing them to shift or drop. Primarily affecting Sino-Korean vocabulary, it transforms an initial r/l (ㄹ) into n (ㄴ) or drops it entirely, and drops an initial n (ㄴ) before the vowel i or y-glide vowels so the syllable begins with an open vowel sound (written with the silent initial letter ㅇ).
Why Do Sounds Shift at the Start of a Word?
Rolling your tongue to launch a word with an r/l (ㄹ) sound, or starting right away with sounds like nyeo or nyo, does not come naturally in Korean.
The Korean language tends to avoid certain consonants at the very beginning of a word, a habit largely rooted in vocal ease and articulatory comfort. When an r/l (ㄹ) lands on the opening syllable, it shifts to an n (ㄴ) or drops out altogether. Likewise, an n (ㄴ) placed directly in front of an i-type vowel drops away so the word opens with a clean vowel sound (spelled with the silent placeholder consonant ㅇ).
For example, in the word namnyeo (남녀, 'men and women'), the second syllable comfortably retains the sound nyeo. However, when that same character stands alone at the front of a word, Korean eases the vocal effort and pronounces it as yeoja (여자, 'woman'). Similarly, the ro (로) in geullo (근로, 'labor') turns into no at the beginning of a word, yielding nodong (노동, 'labor').
Two Main Paths of the Initial Sound Rule
The initial sound rule operates down two distinct tracks. The first track is the transformation of the r/l (ㄹ) sound. When an r/l lands on the front doorstep of a word, its behavior depends on the following vowel. Before plain vowels such as a, eo, o, and u, it converts to n (ㄴ)—turning ragwon (락원) into nagwon (낙원, 'paradise'). Before i or y-glide vowels (ya, yeo, ye, yo, yu), the r/l drops off completely to leave just the vowel sound: ryangsim (량심) becomes yangsim (양심, 'conscience'), and ryeoksa (력사) turns into yeoksa (역사, 'history').
The second track is the dropping of the n (ㄴ) sound. While n readily starts native words like namu (나무, 'tree') or nabi (나비, 'butterfly'), it hesitates before i or y-glide vowels. In these positions, the rule drops the initial n sound entirely, opening straight into the vowel. That is why nikmyeong (닉명) becomes ikmyeong (익명, 'anonymity'), and nyeonse (년세) becomes yeonse (연세, 'venerable age').
Through these shifts, words greet the speaker with comfortable, familiar sounds. This dropping of n was not restricted to Chinese loanwords; native Korean words underwent the same historical shift, transforming archaic forms like nyeoreum (녀름) into modern yeoreum (여름, 'summer') and nima (니마) into ima (이마, 'forehead').
Fine Print: Word Position and Modern Loanwords
To be precise, the initial sound rule applies when two criteria are met: the syllable must belong to a Sino-Korean word, and it must sit at the absolute beginning of the word. From the second syllable onward, the rule does not apply, preserving original readings as seen in sinnyeon (신년, 'new year'), kwaerak (쾌락, 'pleasure'), and toron (토론, 'debate'). (Exceptions exist for specific prefixes, compound nouns, or endings where ryeol/ryul softens to yeol/yul after a vowel or n, such as in biyul [비율, 'ratio']).
Furthermore, because native Korean roots never started with an r/l sound historically, the r/l rule evolved specifically to adapt imported Chinese words to Korean vocal habits. In contrast, modern loanwords from Western languages—such as radio (라디오), news (뉴스), and robot (로봇)—are exempt from the rule in order to respect their original foreign pronunciation.
Interestingly, North Korea abolished this rule under its standard linguistic norms (Munhwa'o). Speakers in the North preserve the original Chinese-character sounds in both spelling and pronunciation, using forms like rodong (로동), nyeoseong (녀성), and rigwa daehak (리과대학). This phonetic divide remains one of the most prominent spoken differences between South and North Korea today.
Common misconceptions
The initial sound rule applies universally to the first letter of every Korean word, including modern foreign loanwords.
The rule applies primarily to Sino-Korean vocabulary. Modern loanwords such as radio (라디오), ribbon (리본), and news (뉴스) do not follow the rule, as their original foreign pronunciation is deliberately preserved.
Where you meet it
The syllable nyeo in namnyeo (남녀, 'men and women') shifts to yeo when starting a word, becoming yeoja (여자, 'woman').
The syllable raeng in geubraeng (급랭, 'flash freezing') shifts to naeng when starting a word, as in naengjanggo (냉장고, 'refrigerator').
The syllable ro in geullo (근로, 'labor') shifts to no when starting a word, becoming nodong (노동, 'labor').
In one sentence
A Korean phonological and spelling rule where an initial r/l (ㄹ) in Sino-Korean words shifts to n (ㄴ) or drops, and an initial n (ㄴ) drops before i and y-glide vowels, creating a smoother start to spoken words.