Devare
A devare (pronounced /dɛˈvɛɘr/) is a planetary world which straddles a neck and hence has two sides opening up on different parts of the containing plane. A lower-dimensional analogue would be a band-shaped world around the narrow middle of an hourglass-shaped two-dimensional chore. The two sides of the band would, to any inhabitants, appear and function much the same as a circular two-dimensional world, but another similar world would exist on the other side of the band, of which the inhabitants of the first side may or may not be aware. A devare is similar to this scenario, but in three dimensions. Another way to visualize a devare is to picture a hollow spherical world, its surface only a rocky shell around a spacious interior, and then imagine the interior of the sphere mipped until it compared in capacity to the outside. This latter picture is a little misleading, however, in that it seems to designate one side of the devare as clearly the outside and the other the inside; in fact, both sides of the devare have equal claim to be considered the "outside", and there is not necessarily any such distinction between them.
The term originated in the cosmos of Doun, in the plane of Piobagh where this type of world proliferates, but it has since come to be used in the æalogical community to refer to such worlds in other planes and cosmoi as well. Similar words have been coined to refer to a world with three "sides" (tribare), four sides, (kierbare), or, generally, two or more sides (rambare). However, in practice these words are rarely used, because such worlds are extremely rare.
Relations between sides
A devare may or may not be connected by natural or artificial shafts; if they exist, such shafts are known as conasters. If the conasters are large and frequently used, the devare may be considered a single world, with little more practical difference between the two sides than between the hemispheres of a typical planet. Otherwise, the two sides of the devare are likely to be considered entirely different worlds, with little traffic between them and with the inhabitants of one side perhaps completely unaware of the existence of the other. In the complete absence of conasters, it may not even be possible to travel between sides of the devare, at least not without translocation or a great deal of circumlocution.
Even when the sides are prominently connected by conasters, it may of course be the case that the inhabitants feel particular attachment and loyalty to their own side, and considerable cultural distinctions may develop between the sides of the devare. It's far from unheard of, in fact, for the inhabitants of the two sides of the devare to be in open warfare against each other, the conasters usually serving as the primary battlefronts.
Formation
Devares tend to form most frequently in planes that due to their topology and curvature are already filled with suitable necks, especially if the local physics leads to a tendency for matter to gather at these necks. All this is, in fact, precisely the case in Piobagh, which is why devares are particularly common there. Similar conditions may exist in other planes which will also likely lead to the formation of devares there. Such natural devares may form gradually by the accumulation of matter, starting as a thin shell and gradually building up as more mass falls into the neck. Some devares made be made partly or entirely of living matter, composed of plants or other organisms that for whatever reason are attracted to a neck.
Devares may also be of artificial origin. A devare may be created by bringing matter to a preexisting neck. Conversely, it may be created by warping the space in the vicinity of a preexisting world, creating a neck where none existed previously. This may mean pinching off a part of the surrounding space, but this is a formidable task; more often, a neck is created by connecting the interior of the world to a different point of the plane (or of another plane), or possibly by expanding the space within the world.
Structure
As would be expected from their means of formation, natural devares tend to be densest in the middle of the shell, equidistant from the surfaces of the two sides. This point (or rather this sphere) also tends to lie at the thinnest point of the neck. The part of the devare closest to the surfaces is generally made of lighter material, corresponding to the outer crust of a terrestrial planet. In some cases, the dense core may contain exotic minerals unavailable elsewhere.
Artificial devares, of course, can have virtually any structure. Some have a similar makeup to natural devares, but others may be more outlandish; devares made entirely of metal with chambers and corridors all through their interiors exist, as do deveres made of glass, bone, and other unusual materials.