English: This figure illustrates world lines drawn in a) positively curved spacetime and b) negatively curved spacetime. In positively curved spacetime, for an initial world line with an event (point) that isn't on the world line, any new world line that passes through the event will intersect the original world line ("parallel" world lines aren't possible). In negatively curved spacetime, for an initial world line with an event (point) that isn't on the world line, more than one distinct world line can be drawn through the event, without any of these new world lines intersecting the original world line. In flat spacetime, exactly one "parallel" world line can be drawn. I, the author of this image, hereby release it for use under the Creative Commons "Share Alike" license. --Christopher Thomas 01:21, 3 July 2006 (UTC)