Alternate Earth
Most æalogists believe that every world exists in a number of alternate versions, and Earth is no exception. As a matter of fact, most alternate worlds presented in the Wongery will be alternate Earths, simply because Earth is likely to be a familiar reference point for Wongery readers. Alternate Earths exist in all the standard permutations: magical Earths, divergent Earths, ferine Earths, and so on. One variety of alternate Earth not generally so common in alternate worlds is the super Earth, an alternate Earth populated in part by characters with various superpowers and, typically, costumes and code names. Naturally, most inhabitants of most of these alternate Earths are completely unaware of the existence of the other alternates; to them, their own version of Earth is simply Earth. (Or whatever they happen to call it.)
Allodeses
While allodeses are a subject that could still bear much further investigation, some allodeses have been tentatively identified that consist mostly or entirely of alternate Earths. Some of these clusters are connected for reasons that are relatively well understood, such as Zanaria, a set of alternate Earths created by a powerful magical phenomenon; and Aulen, a cluster of worlds divided by a god. Others are of more mysterious origin, including Geschichtien, a cluster of divergent Earths branching at various points in history; Long, a cluster of ferine Earths each corresponding to a different animal; and Xerande, a collection of dissimilar alternate worlds that for some reason seem to be connected by a number of portals.
True Earth
True Earth is a common baseline against which alternate Earths are measured. The term doesn't refer to any one particular world, but to a notional world that seems to represent the most common characteristics shared by most Earths. True Earth pertains to the cosmos of Xi; it contains no magic, and no definitely ellogous creatures other than humans. It experienced two world wars in the twentieth century of the Gregorian calendar used by most civilizations there, and so on. Generally, a particular alternate Earth can be presumed to probably match True Earth in any ways not otherwise specified.
The definition of True Earth may be somewhat subjective; it may be that the characteristics that define True Earth don't in fact pertain to most alternate Earths overall, but merely to most alternate Earths that the æalogists who so define it happen to know about. This is particularly true with regard to different allodetic clusters; scholars in other clusters may have entirely different definitions of True Earth.
The Universe
Though it is common to speak only of alternate Earths, in fact the Earths do not for the most part exist in isolation; there may be some cases in which the alternate Earths are self-contained, the only worlds in their own pocket planes, but these seem to be the exception rather than the rule. For the most part, each alternate earth is embedded in its own universe—or, in more unusual cases, in other forms of planes. In most cases, the universe of the alternate Earth is an alternate version of Tamamna, but in some cases the alternate Earth's universe is very different. Some alternate Earths have even been found in entirely different cosmoi.
How much the rest of the universe differs, and to what extent, if any, its characteristics mirror those of the alternate Earth, is a matter that has been so far little investigated, except in certain individual cases. Although it's not entirely impossible that the rest of the universes corresponding to most of the alternate Earths are identical outside of the Earth's direct vicinity, it seems very improbable. More likely, the other worlds of these alternate universes differ as much as do the Earths; they just haven't been explored as much.
Travel between alternate Earths
While most inhabitants of (an) Earth never become aware of the existence of other Earths, let alone travel between them, it is of course possible to move between alternate Earths the same as between other alternate worlds. Aside from the usual means that work to travel between planes, there are a few known aguiae that connect mostly or only alternate Earths. These include the Silver Road, a metallic pathway that appears in isolated locations; Darkwater Cave, a huge cavern system that connects multiple Earths; and Hallashee Hill, a hollow hill with a ring of doors on the interior that each lead out into a different world.