Korean sits here for two peculiarities worth the room: a language split in two by a sealed border, and a number system that keeps two full sets of numbers and makes you switch between them to tell the time. The grammar walkthrough lives in The Morphorian Underground.
Korea is the clearest case anywhere of one language pulled apart by a border rather than by distance. The two Koreas began, in 1945, from a single standard; three quarters of a century of near-total separation has left them with two, close enough to remain one mutually intelligible language, far enough apart that a defector arriving in Seoul has to relearn a good deal of everyday vocabulary. What makes the case unusual is not that it happened, but how completely.
Both standards descend from the same root: the Korean Language Society's unified orthography of 1933 and its standard-vocabulary collection of 1936. The split follows the states. The South kept the Seoul-based 표준어 (pyojuneo, "standard language"). The North built 문화어 (munhwaŏ, "cultured language") on the speech of Pyongyang, and the choice of name was itself political: in a talk of 1966 Kim Il-sung rejected "standard language" on the grounds that it conceded prestige to Seoul.
From there the North pursued language planning of a kind the South never attempted; directed from the top, inside the ideology of Juche, self-reliance. Foreign words were treated as evidence of weakness. A long word-refinement campaign, 말다듬기, replaced Sino-Korean and borrowed terms with native coinages; Chinese characters were removed from everyday use; dictionaries were revised edition by edition to fit the politics, some entries carrying example sentences that quote Kim Il-sung directly. The South planned too, through its National Institute of Korean Language, but moderately, and it never stopped borrowing from English.
The divergence that resulted lives, tellingly, in the vocabulary and the spelling rather than in the deep grammar. Four kinds of difference stand out.
The most visible split. Where the South takes a word from English, the North calques it, coins a native replacement, or, historically, borrows from Russian instead. A tractor is 뜨락또르 in the North, from Russian, and 트랙터 in the South, from English.
Often the North kept an older northern dialect form as its standard: corn is 강냉이 in the North, 옥수수 in the South.
동무 (dongmu) once simply meant "friend"; in the North it became the word for "comrade", on the Russian model, and in the South it took on that political colour and fell out of ordinary use. Even the name of the country differs: the North's Korea is 조선, the South's is 한국.
The headline spelling difference, 두음법칙 (dueum beopchik). The South drops or softens an initial ㄹ or ㄴ in Sino-Korean words, reflecting a centuries-old change in southern pronunciation: "labour" is written and said 노동 (nodong). The North keeps the older initial in its spelling: 로동 (rodong). The surname written Lee or Rhee is the same letter treated two ways: 이 in the South, 리 in the North.
How far apart is that, really? Here the honest answer resists the dramatic figures. Claims circulate that everyday vocabulary differs by thirty to forty per cent and technical vocabulary by more than sixty, and that defectors understand only half of what they hear; these are widely repeated but hard to trace to a published study, and are best treated as impressions of scale rather than measurements (low to medium confidence). Better sourced is the compiler of the joint inter-Korean dictionary estimating that roughly six in ten everyday words are mutually understood (medium confidence). What the defector accounts agree on is qualitative: the friction is worst with English-derived "Konglish" and pop-culture references, and it costs people socially and at work. Some scholars, including Hong Yun-pyo of Yonsei, have argued the gap was overstated during the Cold War, and that some "North Korean words" taught in Southern schools are not actually used in the North.
There was, for a while, a bridge. The 겨레말큰사전 (Gyeoremal-keunsajeon), a joint North-South dictionary of the whole language, was proposed in 1989 and formally launched in 2005, aiming at some 330,000 entries. Work stalled in 2016 as relations froze, and its completion now tracks the politics rather than the linguistics (2018 status well sourced; current status unconfirmed).
Is Korea unique, then? Not quite, and the distinction is worth keeping honest. The mechanism, political isolation driving two standards of one language apart, is exactly the story of German across the inner-German border from 1945 to 1990, of Hindi and Urdu split by script and high-register vocabulary, of Serbo-Croatian promoted into separate national languages after Yugoslavia. In the terms sociolinguists use, these are cases of Ausbau, languages elaborated apart by design, rather than of real structural distance. What is distinctive about Korea is the extremity: an isolation more hermetic and far longer than the German case, where television, mail and family visits crossed the border throughout; a purism directed personally by the head of state inside a totalising ideology; and, most recently, a 2023 North Korean law reported to criminalise South Korean-style speech as "puppet language" (medium confidence), an active effort to widen a gap the others let close. The German precedent is instructive on that last point: after 1990 the two Germanies' vocabularies reconverged quickly, which suggests the Korean divide is held open by the division rather than fixed in the language.
So the fair claim is not that Korean has become two languages; it has not. It is that Korea is the most extreme modern instance of a known phenomenon: one language, still mutually intelligible, pulled about as far apart as politics alone can pull it, with the strain showing exactly where you would expect, in the borrowed words, the coined replacements and the spelling conventions, and almost nowhere in the grammar underneath.
This is a working draft built from a research briefing; several of its figures are flagged above as uncertain, and the load-bearing examples deserve a check against primary standards documents before the piece is called finished.