The working definition
A non-linear book is one whose order of telling departs from the order of the events it tells by design, at the scale of the book: the departures form a pattern the reader learns to expect, not a scatter of local excursions. The definition needs two orders, and the chapter names them plainly. Story order is the sequence in which events happen in the world of the book; text order is the sequence in which the book presents them. The Russian Formalists of the 1920s called them fabula and syuzhet, and every later account of narrative time starts from the gap between them. Any discrepancy between the two is an anachrony, Gérard Genette’s term: a unit told earlier in the text than its place in the story, or later.
A multi-timeline book is a non-linear book whose anachronies are organised as two or more strands. A strand is a line of the telling set in one time, continuous within itself, that the book leaves and resumes; each continuous run of it is an instalment, and each passage from one strand to another is a switch. A strand differs from a cutaway in three ways: it recurs, it advances in its own time between instalments, and it runs on a question of its own, which is the condition the chapter treats as decisive (see The Taxonomy). The interval between two strands is the distance in story time between them at a given point; the rate of a strand is the story time it covers per page. A convergence is the point at which strands meet, and they can meet in three ways: in time, when one strand catches up with the other; by cause, when an act in one is found to produce the situation in another; or by identity, when a person or object is found to be in both.
A non-linear book need not be multi-timeline. A book told backwards, or in fragments ordered by association, departs from story order without strands. The chapter covers both, and gives most space to strands because most revision problems are theirs.
Adjacent terms
Several neighbouring terms belong to other chapters and are used here in the senses those chapters give them. The cutaway is Backstory and Flashback’s single departure (Chapter 7) into the past, with its reach, Genette’s portée, the distance in time between the cutaway and the present it leaves; an interval is the same measure taken between two strands, neither of which needs to be the present. A subplot is The Subplot’s second thread (Chapter 14) with a causal chain of its own; most strands set in another time are also subplots, and a strand shares The Subplot’s tests (Chapter 14) for its chain and adds this chapter’s for its time. The Subplot’s structural convergence (Chapter 14), the point at which threads meet, is the convergence defined above, seen from the side of cause. The transition is The Transition’s join (Chapter 5) between two units, and a switch is a transition that also changes time, usually across a chapter break, which belongs to The Chapter Break (Chapter 6). A strand often has its own viewpoint character, and when it does the switch is also a viewpoint shift, whose costs The Viewpoint Shift (Chapter 13) prices.
An orientation marker is anything that tells the reader which time she is in: a heading, a time-stamp, a tense, a detail of period or wear, an object she has seen before. The term is this chapter’s, and it gathers devices that The Descriptive Detail (Chapter 4) and The Transition (Chapter 5) treat one at a time.
What the chapter leaves to other chapters
The mechanics of the single cutaway, its registers and its exit, belong to Backstory and Flashback (Chapter 7); this chapter begins where the cutaways form a strand or the whole book is out of order. A truth withheld by an arrangement and disclosed late is shared with The Reveal (Chapter 15), which asks whether the withholding has a reason; this chapter asks whether the arrangement has a use. A prologue set in another time belongs to The Prologue (Chapter 10), though a prologue that is the first instalment of a strand is also this chapter’s. A middle told out of order belongs here, and The Middle (Chapter 11) says what it must still do. The order of volumes in a series, and the genres that expect a dual timeline, belong to Series and Genre (Chapter 17).
See also: the full treatment, section 1, on the boundary between a strand and a run of cutaways.