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?

Colors.

When assigning teams or groups, a value neutral way to do it is by color. Red is not intrinsically better or worse than blue, or green. One could make a case that black, white and brown have attached values but if you stick to the established primary and secondary colors you are usually safe.

I like the idea of naming according to sky. The visitor exclaims "Your sky is so red!". Perhaps it was a spectacular sunrise that day or some random event related to dust, but the name sticks: Red Sky. The other timeline is Green Sky, which started as a joke in a newspaper opinion article and caught on. Of course both timelines usually have regular blue skies but sky and color are neutral, and the color labels can be easily adapted to other things. For example, when I visit, I wear my green Hawaiian print shirt to distinguish myself from my red sky counterpart who wears a red Izod.

He is also a little heavier than me, and his teeth have suffered more misfortunes than mine. But he is pretty funny and I have some grilling skills to learn from him.

Named by History

Look for a specific, major divergence in history, and name them by that. For example, one timeline has the North Atlantic Treaty Organization. The other, instead, has the Pacific Defense Conference (formed in the 1930s by the USA, Canada, Japan, Korea, and Australia to oppose the Axis powers of Germany, Italy, and China). So the first timeline is the NATO Timeline, and the second is the PDC Timeline.

Named by Science

A common fiction trope is that alternate timelines and parallel universe resonate on a different frequency. Having an actual, connected example might allow scientists to detect the difference, proving the concept. The two timelines could then be referenced by that value; Timeline 92 nHz and Timeline 83 nHz.

Named by Futurama

In one episode of the show Futurama, there were two parallel universes that interacted (more actually, but it was mainly the two). It was suggested that one be Universe A and the other be Universe B. As you stated in the question, Universe B was not happy with that, so they decided to go by Universe 1 instead. You could do the same thing, or at least something similar, like Timeline α and Timeline あ.

Pick some significant point of divergence, and label the timelines according to the way they went

If the timelines are truly different, then there are going to be things in each timeline that aren't in the other one. So you could name the timelines according to these things.

There are any number of possibilities, depending on what people in each timeline know about the other.

For example, perhaps Picasso had a purple period instead of a blue period. So people might refer to the "Blue Picasso" universe and the "Purple Picasso" universe.

Of course, bigger and more obvious differences than my siily example are very likely to exist, and be better candidates for labeling the timelines. The only limit is your imagination.

I am not so sure your question is far-fetched.

It could be posited that there have always been divergent timelines within human history itself.

Consider the American Civil War, as an example.

There are now two divergent time lines in America. One believes that the Civil War never ended, that the fight still continues, and they have their own heroes, their own version of history, their own concept of a legal system, their own version of reality, their own culture, their own moral code, and in fact in many areas their own judicial system (if you accept that there are Democratic Supreme Court Justices and Republican Supreme Court Justices). The other believes that the end of the Civil War was definitive, that the issues were resolved, and that the legal system resoundingly supports a progressive liberal socialist stance. These two timelines, in fact, have become ideologically dichotomous, to the point where they are incompatible with each other. And the timelines are infused throughout American society and geography. There are Red states and there are Blue states. Each has their own reality, and their own version of history.

The main difference, in your scenario vs this one, is that your timeline has been completely temporally/spatially separated for a period, and then merged back together. This scenario has always been contiguous. Is this separation/non-separation relevant to the result? I posit that it is not. The ideological divide between the two timelines would be just as pronounced, either way, weather they somehow merged after being separated or were always together.

My point is, there is not just one nomenclature for these two divergent timelines. One could just as easily call them "Confederate' vs 'Yankee', 'Democrat' vs 'Republican', 'socialist' vs 'conservative', 'Northerner' vs 'Southerner', and so on and so forth.

There would never be just one designator, one label, one set of terminology. It would depend on what particular aspect of the divide you were looking at.

EDIT

Now, let's go one step further.

Suppose, during the Civil War, there actually WAS a split into two spatially/temporally distinct time lines. One is the timeline we live in. In the second, the South won the civil war. In this timeline, events would have unfolded pretty much along the lines that the 'Confederates' in our current timeline would have liked it to, and who already live to some extent in that reality. Would the factions from each timeline be much different in outlook to their compatriots in our timeline?

So, let's merge the timelines. There are an abundance of people in OUR timeline who would rejoice, and identify with the merging timeline. And there would, undoubtedly, be those from the other timeline who would identify with the prevailing views of people from this timeline. Would there really be a fundamental difference in the ideological divide? Perhaps just more pronounced? Wouldn't the desinators applied in our timeline just be transferred to the factions in the other timeline, and vice-versa? The same expletives? The same pejoratives?

END EDIT

However, there would be, and is, one commonality between both scenarios.

In both cases, the interactions really coalesce into the differentiation 'US' vs 'THEM'. That seems to be absolutely consistent throughout human history.

Name the lines after one of the first differences a visitor sees (in the case where material transportation is possible) or after something different about the format of official messages.

The first-discovered portal on one end is located in a park, and that is the Local Parks Director or Famous Parks Architect timeline. This portal connects to a cavern in the other time line, and that is the Cave Explorer or Prominent Cave Feature timeline.

In one time line, the Comic Sans font is universally loved. In the other, the first entity to make contact uses Garamond font for all official communications.

These names will have history associated with the contact itself and will be items that are obviously related to the travel or communication experience. They have less of a 'judgment on an entire collection of civilizations' character compared to trying to identify the divergent event. And hopefully whoever the landmark or message characteristic was named after has a name that is both catchy and easily pronounceable.

Give them both names that will be equally flattering

"Grand marshal Sikolov of the awesome timeline USA, I'm president Freeman of the kickass timeline USA, I would like to talk to you about collaborating to beat those damn commies...".