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The twin worlds of Symbia and Terra differ from our world, Earth, as follows:

Terra is identical to Earth in history, geography, and everything else, up until the year 2000. After 2000, Terra is Earthlike, but with visits from Symbians, widely spaced (50 or so worldwide) portals between Terra and the matching place on Symbia, and a few other changes (see Changed)

Symbia is different from Terra as follows:

Green sky

Same geography (mountains, rivers, etc) but smaller and more widely spaced cities

No roads (there are "paths" for pedestrians within towns, and occasional "stub" paths leading out of towns to farms or transport points)

No money (see Ard)