“For every Southern boy fourteen years old, not once but whenever he wants it, there is the instant when it's still not yet two o'clock on that July afternoon in 1863, the brigades are in position behind the rail fence, the guns are laid and ready in the woods and the furled flags are already loosened to break out... and it's all in the balance, it hasn't happened yet, it hasn't even begun yet... and that moment doesn't need even a fourteen-year-old boy to think, This time. Maybe this time - with all this much to lose and all this much to gain: Pennsylvania, Maryland, the world, the golden dome of Washington itself to crown with desperate and unbelievable victory the desperate gamble…”
“There are worlds of probability, divergent in the streams of time, but identical almost, until the branches diverge too far…
“Worlds co-existent in time and space - but separated by another dimension, the variant of probability. This is the world that might have been yours had something not happened, long ago. Originally the Dark World and the Earth-world were one, in space and time. Then a decision was made - a very vital decision, though I am not sure what it was. From that point the time-stream branched, and two variant worlds existed where there had been only one before.
“They were utterly identical at first, except that in one of them the key decision had not been made. The results were very different. It happened hundreds of years ago, but the two variant worlds are still close together in the time stream. Eventually they will drift farther apart, and grow less like each other. Meanwhile, they are similar, so much so that a man on the Earth-world may have his twin in the Dark World… The man he might have been, had the key decision not been made ages ago in his world…”
Henry Kuttner, “The Dark World,” 1946