This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.
Set a Janggi board beside a Xiangqi board and the family resemblance is obvious: the same nine by ten grid, the same diagonal palace lines, the same cast of generals, advisors, elephants, horses, chariots, cannons and soldiers. Play the two games and the resemblance thins quickly. The Korean elephant moves in a longer, more restrictive pattern than its Chinese counterpart, the cannon cannot jump over another cannon in Janggi the way it can in some Xiangqi lines, and Janggi allows a pass move with no equivalent in Xiangqi, letting a player simply forfeit a turn when no improving move exists.
The board's most visible difference is the river, or rather its absence. Xiangqi divides its board with a river down the middle, and several pieces, the elephant most of all, may never cross it. Janggi has no river at all: the line was dropped from the Korean game, and with it went the boundary, so the Janggi elephant ranges over the whole board instead of being confined to its own half. The diagonal lines cutting through both palaces rather than one carry a similar loosening, letting the general and advisors move diagonally within their fortress in a way Xiangqi never permits. The result is a game close enough to Xiangqi that a player of one recognizes the other on sight, and different enough, starting with a boundary one game keeps and the other abandoned outright, that fluency in one does not transfer cleanly to the other.
Sources The Oxford History of Board Games with Wikipedia, Janggi