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This file contains background information to the Fifty Worlds campaign setting which everyone knows or can easily find out (i.e. it's the player information). The additional information known only to the game master is here.
As far as is generally known, there are approximately fifty inhabited worlds. No-one is sure of the exact number. Several sages have produced lists of exactly fifty worlds, but they differ. No-one has visited all of the worlds on any list, and no-one is ever likely to.
There are various places where a campaign may start. They all have one thing in common: Newtonian laws are in force. Inanimate objects move in accordance with fairly simple physical laws. Intelligent beings have the additional option of using magic, either by requesting the intercession of their deities or by casting spells gleaned from ancient tomes.
(Historical note: as well as determining several physical laws, Newton also investigated magic, both by requesting the intercession of his deity and by perusing ancient tomes in an attempt to glean spells from them. Check up if you don't believe me.)
(Physical note: whatever anyone else may tell you to the contrary, black holes, wave-particle duality and the uncertainty principle are entirely consistent with Newtonian physics. Relativity isn't, of course. It is an essential feature of this campaign setting that the word now actually means something consistent across worlds which are vast distances apart.)
There are also various means of travelling between the fifty or so worlds. At this point, you don't have access to any of them. You may have heard rumours of magical megaliths, and if you've lived in a city you may have seen arcane obelisks near which wizards appear and disappear, but that's the limit.
This campaign setting was designed for use with AD&D 2nd Edition. Unless your game master tells you to the contrary, assume that you can use any of the core AD&D products, with the following restrictions:
Clerics: the ultra-powerful clerics of the Skills-and-Powers type are on their own. They're either adventuring clerics, cut off from their home base and trying to convert the natives, or heretics expelled from their original faith. There is no organisation behind them. Player character clerics can (and probably should) be clerics of this sort. There are no restrictions other than that the set of powers chosen must make sense and be consistent with the cleric's alignment and deity.
The much weaker clerics of the Priestly-Handbook type also exist. These do have an organisation behind them. Player characters can, if they wish, be clerics of this sort, but I advise against it. Note that on any particular world only a few faiths will have widespread clerical organisations, so the choice is severely limited.
Flying: unless I've read something incorrectly, the AD&D flying speeds are seriously wrong. Assume, instead, that red dragons fly at the same speed as WW2 bombers, and flying carpets fly at the same speed (and with the same manoeuvrability) as WW1 biplanes, and adjust all other flying speeds in proportion. This means that tactics from the aerial combat literature can be used.