Let's say through some unexplained (and largely irrelevant) phenomena, an alternate timeline has suddenly "collided" with ours. The specifics don't really matter besides the idea that the two timelines are now connected. Perhaps it's only possible to send messages from one timeline to the other, perhaps there are portals that allow people and objects to travel from one to the other, perhaps even the two timelines have somehow "merged" to form a sort of patchwork earth with some geographic regions from one timeline and some regions from the other. Again, doesn't matter. Let's also say that our timelines split a few centuries perhaps in the early 1800s.
Now obviously this is major, major news. There are now potentially hundreds of new nations with which to forge alliances and make war. Regardless, the various world leaders are definitely going to want to have a bunch of UN type meetings to discuss the new paradigm. Now one of the first things I'm sure they'll approach is the nomenclature that should be used to refer to things from one timeline and from the other.
For instance, since the split happened in the 1800s it's more than likely there are a bunch of countries that have the same name now. You might have two sets of the united states for instance. They might even have the same flag (apart perhaps from a different number of stars). So what language should you use to differentiate between them?
Well, the "traditional approach" has generally been to talk about timeline A and timeline B, so you'd be able to say A-USA and B-USA for instance. However, there's a problem with this: which timelines get to be A? Which one gets relegated to being B? It might seem like something childish and trivial to care about, but you should never underestimate a nation's willingness to engage in pointless posturing: "If we let them be A, then we'll be marked as weak! We have to make a strong first impression!"
Another idea would be to look to the point where the timelines split and use whatever events did and didn't happen there to label the different versions of history. So, for instance, you could have the "No-napoleon timeline" and the "Napoleon timeline", except of course this might not be such an easily identifiable splitting off point (perhaps the split was a Vietnamese peasant eating breakfast an hour later or earlier which then butterflied its way into radically altering history). Also, going back to the Napoleon example I doubt people would appreciate having to refer to their timeline as the one where some random European dude they had never heard of didn't rise to power.
Here are some criteria that any adequate timeline nomenclature system would seem to have to meet, at least to me (if I'm wrong feel free to correct me and explain why):
- Not arbitrary. Things like timeline-square and timeline-circle are confusing and would probably get too easily mixed up.
- Not too specific. This would be a convention that applies to the whole world (the whole two worlds) so it can't only be meaningful to a subset of it.
- Not preferential. The naming convention can't imply that one timeline is the default, normal or "first" timeline. Things like timeline #1 and timeline #2 are out. As are "normal-timeline" and "bizarro-timeline".
So, what naming convention could be adopted to refer to the two different timelines and the entities they contain?