"There are as many theories as there are chrononauts," said Una. Generally she was quite cagey about her regular line of work; in most cases, to be too talkative was dangerous. But in this strange temporal zone, with its combination of unbreakable linearity and wide-ranging lateral access and the general acceptance of these normally esoteric facts, she felt she could speak more freely than usual.

"Some compare the Multiverse to a vast lake, into which each action is like a stone thrown into water, except in four or five dimensions. Others treat it as a kind of field, in which all of Time and Space exists simultaneously. Some accept the 'branch' theory, but deny the existence of any kind of centre. Regardless of the metaphor one prefers, however, certain constants remain—the dangers involved in passing between temporal zones, the inevitability of echoes across parallels, the importance of being able to camouflage oneself in an unfamiliar zone."