One Character, Two Readings
Type the character 六 into HanMap and you see two very different romanizations side by side: South Korean yuk (육) and North Korean ryuk (륙). The difference is not dialectal drift — it is a deliberate policy choice baked into each government's official orthography.
What the Rule Says
South Korea's 두음법칙 (頭音法則, head-sound rule) reshapes word-initial /l/ and /n/ in Sino-Korean syllables, but in two different ways. Before i or y the consonant drops altogether: 龍 (dragon) is 용 yong, not 룡 ryong. Before any other vowel an initial /l/ becomes /n/ instead: 来 (come) is 내 nae at the start of a word (내일, tomorrow) but 래 rae word-internally (미래, future). The rule was codified in the 1933 Unified Hangul Orthography and reinforced by the 1988 standard still in use today.
Why North Korea Disagreed
When Korea was divided after 1945, the North chose a different linguistic path. The 1948 Joseon-eo Sincheoljabeop (New Korean Orthography) already left the head-sound rule out, and the 1954 Joseon-eo Cheoljabeop (Korean Orthography), issued by the DPRK Academy of Sciences, confirmed its rejection as an artificial distortion of the underlying form. North Korean orthography preserves the initial /l/ and /n/, so the same characters read 룡, 래, 립 — closer to the Middle Chinese source pronunciations.
Political Dimension
The split affects ordinary reading across the border. South Koreans reading North Korean text must mentally re-map dozens of common words, and reconciling the two orthographies is treated as a real task rather than a cosmetic one — linguists from both sides have met to compile a joint dictionary (Gyeoremal Keunsajeon), though progress remains slow.