Worldgate complexes
(NB: The term "complex" is used to describe any group of worldgates that can be demonstrated (by analysis of their relative power or control structures) to be naturally related to one another. The term does not indicate anything about the number of gates: a complex can be as few in number as two, though routinely there are more than that in one of these groupings. Complexes have been seen to have a slight bias toward odd-numbered groupings, groupings based on prime numbers, and groupings of eleven.)
Worldgate complexes occur in two basic forms -- natural and artificial. The second type is often associated with or based on an incidence of the first.
The population pressure required to spawn a worldgate complex from a single parent gate is normally expressed by an equation known as the demobaric formula or gating pressure formula, primarily involving two interacting varables: population concentration per square meter, and and percentage of population expressed as a function of planetary population. (There are other variables of much lesser importance in the equation which, for simplicity's sake, will not be discussed here.) The relationship between these variables is expressed in the threshold constant. Each planet has its own version of the formula and the constant: the relationship between them changes over time as the population of any given planet shifts, as does the way in which new gates may be spawned. For example:
One of the earliest spontaneously-occurring worldgate complexes is at Chur in southeastern Switzerland. Chur grew extremely quickly after its founding in the early Bronze Age, positioned as it was at the crossing of two major trade routes associated with Alpine passes. It was, however, a very small city, so that the inflowing population was very tightly packed inside its walls. When the city's population reached a significant percentage of the Earth's population at that time (the actual amount does not have to be very high to be statistically significant) as balanced against the appropriate concentration of human beings per square meter, it reached the threshold constant, and Chur's first worldgate manifested itself near Welschdörfli. (This also demonstrates a poorly-understood phenomenon called "offset", in which a new gate will routinely appear near a significant population concentration but not actually in the middle of it.) However, Chur's gating complex could probably not have come into being much later than the Bronze Age, as its population as compared to that of the rest of the Earth would then have been too small. The larger the Earth's population becomes, the more difficult it becomes for new gates to manifest, as even very large cities have trouble achieving the threshold constant unless they are skyscraper-heavy. However, once the initial gate appears, later ones may appear (as Chur's Calanda gate did in the 1400's) without reference to lower population numbers: the tendency of a gate to spawn is usually a function of its power structures and the way they interact with the local environment, not the local population's growth or decline. Chur's initial gate being of unusually high power, even for Earth (whose gates tend to be much higher-powered than other naturally-occurring gates elsewhere in this arm of the Galaxy), the "late spawning" is even less of a surprise.
Once a gate has spawned at least once and become a complex, the wizards in the neighborhood who specialize in gating theory and technologies will normally assess it to see how suitable it is to have manufactured or "artificial" gates added on. These gates normally piggyback on the power architecture of the original, spontaneously generated gate or gates; but since they are manufactured, their control structures can be more delicately balanced, aligned and controlled. They can also be built in multiples that can be locked onto specific destinations, or can mirror one another during periods of peak use.