Songbun: a class system the revolution built
An inherited political caste, and a border enforced against people getting out.
What happened
From 1957 every North Korean household was investigated and assigned to one of three broad classes — core, wavering, hostile — subdivided into dozens of categories based on what your grandparents did before 1945. Songbun governs schooling, work, residence, rations and marriage, and it is inherited.
The results
A system founded to abolish inherited class produced one of the most rigid inherited caste systems of the twentieth century — an administrative record with your name in it, unappealable, and paired with a border you may not cross.
Why it happened
Songbun is loyalty made hereditary: a way to pre-sort the whole population by reliability and lock the sorting in place. The revolution did not abolish class; it wrote a new one into the file — because a regime built on permanent suspicion needs to know, by birth, whom it can trust.