with quite a few names."

"You can have it; I'll stick to rockets!" the pilot replied. "Tell me
another thing, though: What's all this about levels, and sectors, and
belts? What's the difference?"

"Purely arbitrary terms. There are five main probability levels, derived
from the five possible outcomes of the attempt to colonize this planet,
seventy-five thousand years ago. We're on the First Level--complete
success, and colony fully established. The Fifth Level is the
probability of complete failure--no human population established on this
planet, and indigenous quasi-human life evolved indigenously. On the
Fourth Level, the colonists evidently met with some disaster and lost
all memory of their extraterrestrial origin, as well as all
extraterrestrial culture. As far as they know, they are an indigenous
race; they have a long pre-history of stone-age savagery.

"Sectors are areas of paratime on any level in which the prevalent
culture has a common origin and common characteristics. They are divided
more or less arbitrarily into sub-sectors. Belts are areas within
sub-sectors where conditions are the result of recent alternate
probabilities. For instance, I've just come from the Europo-American
Sector of the Fourth Level, an area of about ten thousand parayears in
depth, in which the dominant civilization developed on the North-West
Continent of the Major Land Mass, and spread from there to the Minor
Land Mass. The line on which I was operating is also part of a
sub-sector of about three thousand parayears' depth, and a belt
developing from one of several probable outcomes of a war concluded
about three elapsed years ago. On that time-line, the field at the
Hagraban Synthetics Works, where we took off, is part of an abandoned
farm; on the site of Hagraban City is a little farming village. Those
things are there, right now, both in primary time and in the plenum.
They are about two hundred and fifty thousand parayears perpendicular
to each other, and each is of the same general order of reality."

The red light overhead flashed on. The pilot looked into his visor and
put his hands to the manual controls, in case of failure of the robot
controls. The rocket landed smoothly, however; there was a slight jar
as it was grappled by the crane and hoisted upright, the seats turning
in their gimbals. Pilot and passenger unstrapped themselves and hurried
through the refrigerated outlet and away from the glowing-hot rocket.