Every known World has the same diameter, gravity, angle of rotation and length of year as every other. Each has one moon, of the same size and composition as our own. Each is (or originally was) inhabited by human beings, and has flora and fauna of the same basic types (birds, mammals, etc) as our own Earth. All known Worlds share 90% of their flora and fauna in common, and on most the figure approaches 98% thanks to the transmission of species across Worlds. For example, there are many special kinds of animals or birds, but no dinosaurs or trilobites.

Beyond these basic similarities, Worlds differ somewhat: continents and land masses vary in shape and coverage. Stellar constellations and planets, even the surface of the moon, look very different. Geography is inconstant. Some have a single large continent, others have only a few small islands, widely scattered. On all Worlds there are unique animals and plants not found elsewhere. Human cultures show great diversity; no two indigenous societies on different Worlds have ever been found that share languages or cultures. Even human bodies differ: showing different combinations of secondary features and skin tones. At the time of contact, some societies have been in the stone age, others have had fully industrialized nation-states. Most were somewhere in between.

Denayi finds and contacts other Worlds by magic. Trained Initiates draw upon the power of the Circle to send their minds out into what they call the "Void," seeking out the faint traces of other human intelligences. Sometimes they find none. Sometimes they are driven mad. Sometimes they find ghostly echoes of Worlds that might be, or might have been. Very occasionally, they locate a World that seems solid, and return sane to report its existence to their Circle. If they can locate it again, they will examine it to determine its approximate population. From this, they may make a guess at its technology level, and decide whether or not it is worth opening a Gate there. Sometimes they guess wrong, and open a Gate to a world with greater technology than they expected, or find that the world is merely an echo, and that no Gate may be opened.

This (combined with the fact that no two Worlds have ever been found to have more than co-incidental linguistic or cultural similarity) suggests that not every possible parallel Earth exists – even that some force interferes with the actualization of Worlds which are too similar. One popular theory suggests that every World found represents a separate alternate history of Denayi’s own World that branched away some time in prehistory, explaining the wide overlap of species on different Worlds. Yet, none can explain why, if this is so, major land masses and even stellar constellations should differ, as these (presumably) would have become set long before humans evolved. Another explanation posits that Worlds have no common divergence point, but that any World, anywhere, that produced human beings would also produce similar animals and plants, and have a similar gravity and atmosphere. None can say, and most Circle members are not inclined to speculate on such philosophic matters

The Pylon’s appetite is enormous, and Denayi sates it by dominating entire worlds. These worlds can be divided into two categories: those kept for material resources, and those kept to generate souls.

Denayi need some Worlds to produce troops for their armies, to grow food for its people, and to produce the weapons necessary for defense. Worlds reserved for these purposes are usually referred to as Core Worlds. Denayi encourages the production of art and luxury goods in its Core Worlds. Such fertile worlds have many gates linking to each other and to Denayi proper so as to facilitate trade and the movement of armies. The tax on souls in the Core Worlds is minimal

Denayi controls a much larger set of Worlds that they harvest for souls. On most worlds, it prefers to take a percentage of the population each few years. In the past, however, it has   systematically "harvested" entire Worlds every two to three generations, leaving only enough behind to repopulate it.