The geometric terms flat, opened, and closed, depend on the overall amount of mass in the universe. Just the right amount and the geometry of the universe is such that parallel lines will remain parallel. Lesser amount of mass and the lines will diverge. Over the amount and the lines will eventually converge. Flat is the first scenario. Opened, the second, and closed, the third. You can think of flat as a cubical grid system, like a Rubrics Cube.
but spherical in all directions X,Y and Z?