Here, I contrasted alternative history fiction set sideways in time with cosmic fiction set futureward along a single timeline. Of course, fictional futures are mutually incompatible and are overtaken by the advancing present. Thus, they become alternative timelines even though they were not originally conceived as such. This process occurred with Poul Anderson's first and second future histories. First, they are mutually incompatible, different histories. Secondly:
Sandra Miesel's introduction and interstitial additions to Anderson's Psychtechnic History identify 1958 as the year of World War III and thus as the divergence point between that history and ours;
Nicholas van Rijn demonstrates that the Technic History is just one of many histories by visiting the inter-cosmic inn, the Old Phoenx.
In the latter case, van Rijn drinks beer with Eric the Red and Sancho Panza. Thus, the landlady must interpret between early Norse, a Spanish peasant dialect and what is described as a future spacefaring argot. We know that Anglic, League Latin and extraterrestrial languages like Planha and Eriau are spoken in van Rijn's time. Out of all this, spacefarers would concoct an argot.
I am about to drive up to Cumbria so will sign off till later.