A plane, in the terminology used by most æalogists, is a continuous expanse of self-contained space; that is to say, one that doesn't physically connect to any other such expanses. There's a little leeway given if the connections are very small; two large planes can be physically connected through a narrow wormhole and still be considered separate planes. This may seem to lead to ambiguity concerning where exactly the line is drawn, and just how big the connections can be between two planes before they have to be considered one, but in practice such borderline cases very seldom arise; for the most part, it's pretty clear what are and are not separate planes.
Even within a cosmos, planes may vary widely, not only in their contents, but in their sizes, their shapes, and, to a limited degree, even their physical laws. Of course, planes with entirely different physical laws would by definition exist in different cosmoi, but the physics of some cosmoi allow for a certain amount of variation—and even where it doesn't explicitly allow for such, there may still be effective variation due to magical effects. Thus two planes may differ in such factors as the rate of flow of time, the strength and direction of gravity and other forces, et cetera.
Generally, the unmodified word plane is reserved for relatively large expanses—hundreds of kilometers across, at the very least. Smaller self-contained spaces are called by other terms such as "pocket planes". Æalogists don't hold to this distinction, and technically any self-contained space can accurately be called a plane, but in common usage other terms are still used for particularly small planes.
There are other terms that are in common usage to refer to planes, as well. In many places, planes are often informally referred to as "dimensions", though this usage is frowned upon by æalogists, not least because of its possible confusion with the geometrical concept of a dimension. Of course, the word "plane" has its own geometrical meaning at variance with the æalogical one, referring specifically to a flat two-dimensional expanse, whereas most planes in the æalogical sense—or at least most known planes of interest to æalogists—are, or at least locally seem, three-dimensional. Because of this, some æalogists prefer to eschew the word "plane" as well, at least in technical contexts, and favor more specific terms such as "perioch" (pronounced /ˈpɛriːɒk/; from the Greek περιοχη), "singulum" (plural singula; from Latin, and not to be confused with the homophonic "cingulum"), or "chore" (which is pronounced like "core" and perhaps exacerbates the issue by having both a homograph and a homophone; from the Greek χωρος). Such terms, however, have in few places caught on outside of scholarly contexts, though they are occasionally used when the word "plane" might lead to ambiguity. The phrase "plane of existence" is also sometimes used to avoid this ambiguity, and to refer exclusively to planes in the æalogical sense, though the phrase is more often used by laymen than by specialist æalogists.
Shapes of planes
Despite the name, a plane is not necessarily flat. Some infinite planes may indeed be realmar, though they need not be; infinite three-dimensional planes can have irregularities of curvature, and can include any number of handles and phaci and synammata and other exotic features. It's also quite possible for them to be infinite along only one or two dimensions, making them trochilical or phalaggomatic. Finite planes, too, may occur in many different topological shapes, from glomes to ungules to ploces to shapes still more exotic. Such shapes may be detectable only on the very large scale, and have no perceptible effect on the average world. However, if the curvature of the plane is great enough, at least in local volumes, it may have significant repercussions; in a suitably curved plane, it's possible to have worlds of toric or scambic or other unusual shapes.
While most planes under discussion in the Wongery are three-dimensional (or at least seem three-dimensional on a macroscopic scale), this need not be the case. Planes of four or more dimensions may exist, populated by correspondingly higher-dimensional beings; even an inhabited two-dimensional plane is conceivable, although one dimension may not allow enough complexity for life. (A flat two-dimensional plane of existence is, of course, a plane in both the æalogical and the geometric sense.) However, conditions on these other-dimensional planes will necessarily be radically different simply due to their geometries, and traveling between planes of different dimensions may be as difficult as traveling between different cosmoi, if not more so, and for much the same reasons.
Planes may or may not be embedded in a higher-dimensional manifold. If they are, their arrangement can vary; they may be geometrically parallel, or skew. To use a lower-dimensional analogy, consider two-dimensional worlds, perhaps in the shapes of the surfaces of spheres. (While a sphere itself is three-dimensional, its surface is two-dimensional, albeit not flat.) These spheres could be entirely discrete and separate, or they could be embedded within a three-dimensional manifold. In the latter case, the spheres could be offset from each other, or they could be nested one inside the other; if the three-dimensional manifold is itself not flat, it's possible for the spheres to be essentially "nested" even though they're of equal size (just as it's possible for circles to be so arranged on the surface of a cylinder or a tore).
In general, though, the actual geometrical arrangement of the planes is unimportant to the inhabitants of the planes, and even undetectable by ordinary means. Whether the planes are parallel, or at some strange angle, or even intersecting, rarely matters unless there is some means of interplanar travel that depends on the planes' physical proximity, or unless a plane can produce some kind of changes in another plane close to it. Such effects may, however, be determined by other factors than geometrical distance.
Sometimes planes are said to be "adjacent", or "nearby"; this merely means that it is easier to travel between these planes than between more "distant" planes. This could mean, for instance, that spells that allow interplanar travel are easier to cast when used for travel between the "adjacent" planes, or perhaps some spells work for travel between those planes and not between others, or perhaps there is an abundance of portals leading between the adjacent planes. Instead of or in addition to the ease of travel between them, planar adjacency may be a measure of how much the planes affect each other; if events or conditions on one plane tend to somehow "leak onto" and influence events and conditions on another plane, these two planes may be considered to be adjacent regardless of whether or not one can easily travel between them.
In any case, this adjacency may occur from a variety of reasons, and does not generally mean that the planes are actually geometrically close to each other (which in any case depends on the metric used for measurement, and may be an altogether meaningless proposition if the planes aren't embedded in a higher-dimensional manifold). Based on adjacency, maps are sometimes made of the arrangement of planes within a cosmos or chorodesis, and the planes are sometimes said to be arranged in a cube, or a simplex, or some similar shape. This is only a figurative representation of their relative adjacency, and does not necessarily imply that the actual geometry of the planes resembles these shapes, or higher-dimensional analogues. A three-dimensional model of such a planar arrangement (or more generally a model possessing the same number of dimensions as the planes it represents) is called a iuchurus, or a planar orrery. Elaborate clockwork iuchuri exist to represent sets of planes that change their arrangement over time.
Sometimes a set of planes is lumped together into a larger grouping called a chorodesis. Most often, the unifying factor is their adjacency, the fact that it is significantly easier to travel between the planes within the grouping than to or from planes outside it. However, planes may also be collected into a chorodesis on the basis of a similar origin, or similar conditions, or even similar inhabitants. At times multiple chorodeses are also collected together into a higher-level collection sometimes called a detodesis. Some æalogists hold that a cosmos is just a special type of chorodesis (or detodesis), united by similar physical laws.
Types of plane
Although the possible variety of planes is truly vast, æalogists do often find it useful to define some categories. They therefore classify planes into different types according to their contents, origins, uses, and relations to other planes. This list of types of plane should not be thought of as an exhaustive categorization. By no means does every plane fall into one of these types, and many planes belong to two or more types at once—alternate planes, for instance, are very commonly isomorphic (and vice versa), and spiritual planes (especially divine planes) are frequently eschatological... and often also propositional and/or psychic. Not all cosmoi contain all these sorts of planes, and none of these sorts of planes is known to exist in all cosmoi (with the possible exception of terrestrial planes, but it may be simply that cosmoi without terrestrial planes haven't been discovered because they are inimical to normal life).
An adaptive plane is a plane that transforms all those who travel there according to some system. It may transform all ellogous creatures into beasts or sonders, for instance, or transmute all entering creatures into a particular substance. (The latter may be the case in an adaptive elemental plane.) There may be means of avoiding the "adaptation" when traveling to the plane, but if so they will be relatively difficult. The adaptation may reverse automatically upon leaving the plane, or it may be permanent; in the latter case, it's not impossible in some cases that if one leaves the plane and reenters the adaptations will accumulate.
An alternate plane is a plane that bears significant resemblance another plane, called the base plane, but is changed in certain important particulars. Alternate planes may be superficially identical to the base plane, distinguishable only on close examination, or their differences may be obvious and far-reaching, while still leaving sufficient similarity to qualify them as alternates. There may be more than one alternate to the same base plane, although which is the alternate and which is the base may be somewhat subjective, and some æalogists prefer to do away with the concept of the base plane altogether, and simply speak of planes as being alternates of each other.
An artificial plane is a plane intentionally (or perhaps subconsciously) created by one or more ellogous organisms. Some such planes bear obvious signs of their manufacture, being made of clearly fabricated materials, such as brick and plaster and fitted-stone walls. Others, however, may be indistinguishable from naturally occurring planes by anyone unaware of their histories.
A barrier plane is a plane that impedes access to one or more other planes, preventing travel to the blocked planes without passing through the barrier plane first. Barrier planes are often artificial, set up intentionally to keep interlopers out of a protected area—or to keep dangerous denizens in. Such planes are likely to be filled with traps and perils and all but impassible. In some cases, however, a barrier plane may occur naturally; such a plane may also be hazardous and inhospitable, but may also be a harmless plane that at worst only inconveniently lengthens an interplanar journey.
A carceral plane is a plane used for the confinement of prisoners, whether those prisoners are committed there as punishment for some (real or imagined) crime or because they are terrible monsters from whom other worlds must be protected—or perhaps simply at the cruel whim of some capricious custodian. Some carceral planes were expressly created for that purpose, but others are naturally occurring planes that have been coöpted into planar prisons.
A cellary plane is a plane used for storage of goods. Most cellary planes are small pocket planes used exclusively by single individuals, or small parties, but larger cellary planes may exist with many separate compartments, or with their volume otherwise divvied up. Those storing their effects in a cellary plane often take extreme measures to prevent travel into the planes and protect their contents from theft or sabotage.
A dejective plane is a plane used for the disposal of unwanted objects, or the exile of banished creatures. Retrieval of objects cast to a dejective plane is generally difficult, but not necessarily impossible. Dejective planes are rare because in general it's much easier to simply destroy an undesired object than to transfer it to another plane, but they may exist in cases where there is some very simple spell or paracarminical means to cast an object to a particular plane, or where some easily accessible one-way portal leads to it.
A dyschronic plane is a plane in which time passes at a different rate, relative to some reference plane (often the home plane of the speaker). If time is faster, the plane is hyperchronic; if slower, hypochronic. Naturally, whether a plane is hyperchronic or hypochronic, and indeed whether it is dyschronic at all, depends on the reference plane. If time passes at the same rate on Plane A and Plane B, but faster on Plane C than on either of the first two, then Plane B is hypochronic relative to Plane C and isochronic (that is, not dyschronic) relative to Plane A, while Plane C is hyperchronic relative to Plane B.
An elemental plane is a plane associated with a particular material. These components may or may not correspond to the fundamental components of matter; more often, they correspond to some magically defined materials which are not physically fundamental. Elemental planes are composed mostly, if not entirely, of their associated material; if they have native life, those creatures too will be made of these materials. The difficulty of traveling through and surviving in an elemental plane may depend on the element; a plane associated with air may be easy to survive in (as long as one brought a supply of food and water), whereas one associated with stone may be made of solid rock and impossible to traverse by nonmagical means, and one associated with magma may be too hot to bear. However, sometimes these factors are counterintuitive; it could be that in a particular cosmos or chorodesis the plane of air contains hurricane-force winds that constantly bombard its contents with deadly force with sharp particles of foreign matter, while the plane of stone is permeated by airy tunnels that make it easy to survive there.
An erogative plane is a plane from which creatures and objects are summoned to other planes, especially to terrestrial worlds. An erogative plane may otherwise seem rather terrestrial itself, with its contents and inhabitants going about a rather ordinary existence until and unless they are summoned. This is not, however, necessarily the case; it's also possible for the contents of an erogative plane to be held in some sort of suspended animation until they are needed. It's also possible for the objects summoned from an erogative plane not to physically exist at all when not summoned, but for the summoning to shape them somehow out of the plane's forces and contents; in this case, the erogative plane
may act less as a repository for summonable objects than as a factory to produce them.
An eschatological plane is a plane that serves as the final resting place for the souls of dead mortals, or as the intermediate resting place if they are destined for reincarnation. Often eschatological planes come in chorodeses, called aedodeses, with the particular plane a specific soul goes to decided by his behavior in life, or by the god he worshipped, or by other factors. There may be one eschatological plane, or set of related eschatological planes, for an entire cosmos, or different worlds—or even different areas, species, or cultures within a world—may have their own such planes or aedodeses.
Some groups of eschatological planes are subdivided into celestial planes (heavens) and infernal planes (hells), according to whether they are planes of reward for the righteous or punishment for the guilty. However, not all sets of eschatological planes can be so subdivided; there are many cases in which all the dead from a particular world go to a single eschatological plane regardless of their behavior in life, or where their disposition is decided along entirely different criteria from their "righteousness" (such as how they died, what burial preparations were made, or even their names); even where there are such planes of reward and punishment, there may also be some intermediate planes (sometimes called limbos) for those not worthy of the former but not unworthy enough to merit the latter.
Sometimes referred to as perindes, isomorphic planes are pairs of planes that can be mapped into each other in some continuous and meaningful way. That is, first, the two planes must be topologically homeomorphic; it must be possible to map each point on one plane to a distinct point on the other in a continuous and invertible way. Furthermore, for at least one such mapping, there must be some significant correspondence between points. This may mean, for example, that points that contain land, water, or air on one plane contain land, water, or air on the other, and those containing water on one plane contain water on the other; it often goes deeper than that, however, such that corresponding areas of the two planes show marked similarities. If distances are preserved across two isomorphic planes, the planes are said to be congruent.
An object plane is a term used to describe the plane mirrored by an umber plane, or one of the planes a transitional plane is transitional between. Referring to a plane as an object plane does not imply anything in particular about that plane itself; it merely means that there exists an umber plane that mirrors it, or a transitional plane between it and another object plane.
A pocket plane is simply a very small plane. This is not a technical æalogical term, and there is no firm cutoff for whether or not a plane qualifies as a pocket plane; one person might refer to a plane large enough to hold a typical planet as a pocket plane, while another may reserve the word for a plane smaller than a city.
A propositional plane is a plane associated in some way with some sort of abstract principle. These principles may include emotions, personality traits, or such lofty concepts as good and evil. Propositional planes seldom occur singly, but usually in chorodeses with planes representing related principles. Exactly how the planes embody these principles depends on the cosmos, but often their nature affects not only their native contents, but also visitors to the planes.
A psychic plane is a plane which does not seem to exist in material form, and which cannot be physically entered, but which can be visited in some mental or spiritual sense. This may be because it actually exists on another cosmos, or because it does not support ordinary life. A psychic plane may have inhabitants to whom it seems "solid"; it would not, then, be a psychic plane from their perspective. It may be possible for there to exist some very difficult or convoluted means of visiting a psychic plane physically, but as long as the usual way of visiting the plane remains mental or spiritual, it is still considered a psychic plane.
Spiritual planes are the home planes of various kinds of magical beings. They can be subdivided according to the type of spirit dwelling there; the home planes of gods are called divine planes, whereas the home planes of demons are demonic planes—though of course it's quite possible in some cosmos or chorodesis for gods and demons to be native to the same planes, or for either or both to have arisen on some terrestrial world. New such entities may or may not be continually coming into being on the spiritual planes; they also may arise from deceased mortals (if the spiritual planes are also eschatological), or they may procreate normally (among other possibilities).
Structural planes are planes that exist as part of the necessary infrastructure of a particular cosmos or chorodesis. They exist because something about the local physical laws requires them to exist, either per se or as a corollary to the existence of certain other planes. Structural planes may or may not be inhabited, and often may be put to use as cellary planes, as viatic planes, or for some other purpose.
A subjective plane is a plane that changes according to the thoughts or will of creatures within it. Some subjective planes require varying amounts of conscious effort to change them; others may change due to creatures' subconscious notions or desires, which may mean that it isn't initially obvious why they're changing at all. In some cases, changing the plane requires a certain sort of mental discipline that must be learned or developed, although then the difference between this method of changing the plane and magic may seem ill-defined.
A terrestrial plane is a plane that supports Euterran life, such as humans, plants, et cetera. Not every point in a plane must be able to support such life for the plane to qualify as terrestrial, as long as a significant part does, even if that significant part is small compared to the total land surface of the plane; Tamamna, for instance, is considered a terrestrial plane even though habitable planets like Earth make up only a minuscule fraction of its volume. Æalogists disagree as to whether the perceived commonness of terrestrial planes is real or merely a factor that terrestrial planes are easier for terrestrial creatures to find.
A transient plane is a plane that passes in and out of existence, whether it be according to a regular schedule or at unpredictable intervals. What happens to non-native contents of the plane when it disappears varies; they may be shunted off to another plane (either a fixed plane, or at random), or they may cease to exist along with the plane until it reappears... or perhaps permanently.
A transitional plane is a plane that arises between two other nearby planes, and is in some way intermediate between them. The exact factors that lead to the formation of transitional planes depend on the particular cosmos. When traveling between two object planes that have a transitional plane between them, it is often necessary—or at least simpler—to pass through the transitional plane on the way. (If this is the case, then the transitional plane is also a barrier plane, but not all barrier planes are transitional.) It is entirely possible for more than one transitional plane to exist between two planes, either corresponding to different parts of the object planes, or sequential such that one must pass through the entire series of transitional planes to go between the object planes.
An umber plane is a plane that at some level mimics another plane. This doesn't mean that it is an exact duplicate of that other plane, but that events on one plane directly affect events on the other. Umber planes may resemble closely their object planes, or they may look superficially completely different but have features and contents that correspond to certain significant features and contents in the other. Umber planes are frequently isomorphic, but not always; the reflected features may have an entirely different spatial relationship in the umber plane as in the object plane.
A universe is a plane containing a number of similar mounds, or planetary worlds. These mounds may or may not be physically connected, and there may or may not be simple means of traveling between them. At least some of the worlds in most known universes are terrestrial worlds, leading to those universes being considered terrestrial planes, but a non-terrestrial universe solely comprising alien and uninhabitable worlds is not inconceivable.
A viatic plane is a plane most commonly used for travel between other planes, either because it is adjacent to two or more planes not adjacent to each other or because it is directly linked to those planes through portals or some other interplanar connection. This does not mean that the plane cannot also have other purposes, only that there are at least some individuals who see it largely as a way to get between other planes. Whether or not a plane qualifies as viatic may depend largely on one's point of view; certainly no plane is viatic to its own residents, but even for those not native to the plane one group may see it as a viatic plane while another sees it as better suited for a different purpose entirely.
Wandering planes are planes that vary in their arrangement relative to other planes. A wandering plane may be adjacent to one particular plane or set of planes at one time, and then some time later be distant from those planes and adjacent to an entirely different group of planes. It may even be considered to shift between belonging to different chorodeses, depending on which planes it is adjacent to at a given time. This does not necessarily mean that the plane is literally moving through some higher-dimensional space; while this may be the case, it may also be that it is the metric of the space that is changing, or that the planes' proximity depends on some external factor not directly related to the geometry of any manifold in which the planes are embedded.
Travel between planes
Because planes are by definition physically discontinuous from each other, it is not generally possible to travel between them by conventional means. The definition of a plane does allow some small connections in the form of wormholes, which may provide a simple way of getting between the planes. Furthermore, in many cases there exist interplanar portals that, while technically an entirely different sort of connection than a wormhole, in practice often works much the same way. Such wormholes and portals connecting planes may occur singly or in groups or clusters. Indeed, clusters of portals are common enough to warrant terms to refer to them. A place with a number of portals to different worlds is a compit; if there's little there except portals to different worlds, and if the compit exists outside any particular world, it's called an aguia. A special case is a city with portals to many different worlds, which is called a millevia.
While portals and wormholes are accessible to anyone with no additional aid, if no such passages are available, there are other means of travel. Some spells and talismans can translocate people and objects between planes, as can some technological devices, though the exact mechanisms vary. Similar means can also produce portals or wormholes, either temporarily or permanently. There are even people called golthoi who spontaneously produce portals around them, though the origin of their power is unclear.
Creating new planes
Many planes exist naturally, being an innate part of the cosmos where they are located, and have apparently existed as long as the cosmos. However, it is also possible for new planes to come into existence, a process called chorogenesis. Sometimes planes are generated naturally by physical or magical processes, but planes can also be artificially created.
Creating a plane, however, generally takes immense power, and is not the purview of most ordinary people. Gods and the most powerful mages may be able to create a plane almost on a whim, but for most people it would be a much more difficult process, though one that may be aided by suitable powerful talismans or devices. The creation of a plane may involve also designing its contents in detail, or may leave that up to some sort of procedural production.