What was used as the border between North Korea and South Korea?

CORRECT

Image in the pubic domain.

38th Parallel

When Japan lost World War II, Korea was divided into two zones. The Soviet Union took control of the north and the United States the south. They were divided at the 38th parallel. This was meant to be a temporary division, but it is still there today. North Korea is the country that is north of the 38th parallel.