A novel sequence is a set or series of novels which share common themes, characters, or settings, but where each novel has its own title and free-standing storyline, and can thus be read independently or out of sequence. A novel sequence contains story arcs or themes that cross over several books, rather than simply sharing one or more characters.
There is no useful, formal demarcation between novel sequences and multi-part novels. Novels that are related may or may not fall into a clear sequence. It is also debatable whether a trilogy is long enough and whether its parts are discrete enough to qualify as a novel sequence.
For example, the Barchester novels of Anthony Trollope are only loosely related, although they contain a recurring cast of characters; his political novels about the Pallisers have a tighter connection and dynamic. A strict definition might exclude both.
The novel sequence was a product of the nineteenth century, with James Fenimore Cooper's works appearing in the 1820s, and Anthony Trollope's Barchester books in the 1850s. In French literature, Honoré de Balzac's ambitious La Comédie humaine, a set of nearly 100 novels, novellas and short stories with some recurring characters, started to come together during the 1830s. Émile Zola's Rougon-Macquart cycle is a family saga, a format that later became a popular fictional form, going beyond the conventional three-volume novel.
A roman-fleuve (French, literally "river-novel") is an extended sequence of novels of which the whole acts as a commentary for a society or an epoch, and which continually deals with a central character, community or a saga within a family. The river metaphor implies a steady, broad dynamic lending itself to a perspective. Each volume makes up a complete novel by itself, but the entire cycle exhibits unifying characteristics.
The metaphor of the roman-fleuve was coined by Romain Rolland to describe his 10-volume cycle Jean-Christophe. In the preface to the seventh volume, Dans la maison (1908/1909) he wrote: "When you see a man, do you ask yourself whether he is a novel or a poem? ... Jean-Christophe has always seemed to me to flow like a river; I have said as much from the first pages."
The term has subsequently been applied to other French novel sequences, particularly of the years between the world wars, notably:
The 19th-century predecessors may be distinguished as being rather "family sagas", as their stories are from the perspective of a single family, rather than society as a whole.
Proust's work was immensely influential, particularly on British novelists of the middle of the twentieth century who did not favour modernism. Some of those follow the example of Anthony Powell, a Proust disciple, but consciously adapting the technique to depict social change, rather than change in high society. This was a step beyond the realist novels of Arnold Bennett (the Clayhanger books) or John Galsworthy.
The twenty-novel Aubrey-Maturin series by the English author Patrick O'Brian has been called perhaps the best-loved roman fleuve of the twentieth century: "[an] epic of two heroic yet believably realistic men that would in some ways define a generation".
The introduction of the preconstructed novel sequence is often attributed to E. E. Doc Smith, with his Lensman books. Such sequences, from contemporary authors, tend to be more clearly defined than earlier examples. Authors are now more likely to announce an overall series title, or write in round numbers such as 12 volumes. These characteristics are not those of the classical model forms, and become more like the franchises of the film industry.