If sentences may thought of as the bones of a story, the humble paragraph provides the muscle that gives a story strength and endurance. Artistic strength and endurance come from another source; paragraphs provide the technical construction and govern the approachability of a story. Paragraphs hold the reader’s attention during a scene and lead him to important plot points. Paragraphs provide order for description, and cohesion for exposition.
As far as the reader is concerned, approachability is the single most important factor in your writing. The reader wants your story to be easily understood and digestible in bite-sized chunks; part of this comes from your lexicon, but just as much of it comes from your paragraph construction. Paragraphs visually break up the text on a printed page, causing the reader to take a brief mental break from his reading and process the information just presented.
Matching subject breaks to paragraph breaks ease the reader’s transition. Every time the subject of a description, exposition, or dialog changes, a new paragraph should be started. Dialog provides the obvious example; paragraphs change when speakers change. Exposition tends to break up fairly naturally as well; we are accustomed to dividing our thoughts and information processing according to subject matter. Description provides the greatest challenge for the writer, because the subject of the paragraph can be different from the various subjects of its separate sentences.
Paragraph subjects need to be divided into naturally discreet portions in order to avoid following a subject through several descriptions or narratives without a break. The writer may avoid this pitfall by employing one of the Rules of 4: a paragraph may devote up to four descriptive, narrative, or dialog sentences to a subject, and may mix these with up to two sentences of one other type. This rule means the paragraph breaks whenever it reaches six sentences, whether it needs to or not. This point is technically and gramatically arguable, but is employed primarily to retain the reader’s attention and promote the visual appeal of the text.
Lengthy paragraphs intimidate the reader and tire him out, forcing more information to be retained and processed in large bites. Counter-intuitively, paragraphs that are too small have exactly the same effect, with the added reader perception that the text is disjointed and choppy even if all of the sentence share a subject. Using a single sentence as a paragraph is usually okay, provided it occurs predominantly in dialog. The reader will forgive much and occasionally embrace short dialog exchanges, which are seen as “snappy” and “witty”.
Double sentences are discouraged, as readers are accustomed to finding thoughts in groups of three: premise, argument, and conclusion. The double sentence paragraphs presents only a portion of this trifecta, leaving the reader unconsciously aware that something is missing. This means it is desirable to use shorter sentences than necessary when writing description or exposition if it will extend a too-short paragraph to three to four sentences. The purpose of the paragraph construction is to present the reader with information in cohesive portions which he will recognize as such.
The reader is entirely at the mercy of the writer for the duration of a story. It becomes the writer’s responsibility to govern the flow of words in order to stem a deluge and stave off a drought. The reader cannot enjoy a story if he has been dried up or drowned.