The setting has done the job.
Multiplot, multiple-viewpoint novels often achieve a similar feeling of unity almost entirely by reliance on common setting as the binding factor. The suspense novels of writers like Tom Clancy and Clive Cussler rely heavily on same-setting unification. A few years before these writers attained their present popularity, Arthur Hailey made unifying setting the bedrock foundation of his novels lik e Hotel and Airport. In the novels of Phyllis A. Whitney, setting is always an important unifying factor as a bewildering variety of characters assail and confuse the first-person narrator; further, in the Whitney novels, the setting very often includes hidden history—past events concealed by some of the characters—whose eventual revelation is central to working out of the plot and the main character's personal problems.
Consider, for example, Cussler's breakthrough novel, Raise The Titanic, in which the long-sunken wrecked ship is constantly at the center of discussions, maneuvers, plans and counterattacks. If the Titanic were not at the heart of the setting as both focus and target of everyone's quest, the dozens of viewpoint changes would be hopelessly confusing.
Or consider Hotel, in which the very purpose of the novel is to place a wildly mixed batch of characters and plot problems within a single setting and show how the problems all work out within that unifying setting.
In the case of Whitney, try to imagine how a novel such as The Trembling Hills could work at all if the setting were not San Francisco at the time of the great earthquake there. Without the unification of constant references to that colorful historic setting, the multiple story lines would seem to "fly all to pieces" in apparent lack of relevance to one another.
Let me encourage you to study a number of recent novels of your own selection; look at the setting and examine the different viewpoints and plot lines. Ask yourself how many of the varying elements tie directly to one another in ways other than through reference to the setting. I think you will be surprised to see how often divergent aspects of plot and character would not be seen as related at all if they did not play against identical setting backdrops.
Further, you might want to consider how the tone, mood and atmosphere of setting will unify a story. From Edgar Allan Poe to Stephen King, horror writers have known this to be so. The consistent emphasis on darkness, dankness, isolation, eccentricity and occult intervention gives King's novels, for example, a unifyingly frightening feel that no single plot element or character can provide. To put this another way, in the novels of King and other horror writers, sometimes the consistent feeling of dread and fright comes not so much from what happens as it does from where it happens, and how that place feels.
Thus you can not only make your story more believable and convincing through sound use of setting, you can also unify it both in terms of making disparate plots and characters seem related and in terms of building up a story atmosphere which will cloak all characters and events within a single feeling matrix.
This is another reason why setting is so important, you see. You get not only obvious credibility advantages from proper handling of setting, but also unification of other story elements.
UNIFYING TECHNIQUES
A variety of techniques are available that will help you use your setting as a unifying "binder." We will look at six of these techniques, those used most often by writers to create a strong sense of cohesion in their stories. Study each technique, and use them to unify your own stories.
Consistent and repeated reference to a single aspect of your setting will keep that aspect uppermost in your reader's mind. Then, as you show different characters noticing this single aspect, or as you play out different scenes near it, or include a reference to it, you consistently remind the reader that, "Hey,
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