[PTTG]

A skerry of a dozen worlds, each radically different from the next. Some are high mana, most no-mana. In some, psionics works, in other not at all. Two have weird-science-based ultra-tech, and one has normal modern tech and Normal mana.

Despite the deep structural differences, the history, language, and population of all twelve worlds is identical. Each world has, as far as anyone can tell, an exact one-to-one correlation of people with the same genetics, name, and overall personality on each world.

Besides the massive changes in physical laws, the biggest difference is that people do not have the same social roles from world to world. A king in one world has a duplicate who is a pilot in annother, a doctor in a third, and a farmer in a fourth. These worlds stay in lockstep, and if an assassin named Darso stabs a king named Narid in a medieval world, a Doctor named Darso will accidentally kill a patient named Narid with a scalpel in a modern world, a pilot will die in an argument with a baggage handler, and a farmer will fall on his neighbor's scythe.

Visitors to any of the worlds must be extremely careful. Nobody knows what will happen if they get involved in history.