“Paragraph: Currently, a piece of writing or print of variable length and having a variety of internal structures, arranged as a single block of text. It can contain only one sentence, but generally consists of two or more sentences presenting an argument or description. The beginning of a paragraph is usually indented in print, unless preceded by an interlinear space, but not always in handwriting or word processing, not in display material. Sometimes, both indenting and extra line space are used to make each paragraph stand out strongly.
The layout of texts in European languages has changed considerably since the Middle Ages, when the paragraph was not a consistently organized unit of prose, and prose was not a highly developed form of writing. The development of printing in the 15th century encouraged the use of paragraphs in blocks of lines that could be manipulated easily by the printer and helped break up the appearance of page after page of print. However, balance in the presentation of lines of print, whole pages, and the effect of the message has been a minor consideration in teaching composition and in the development of print. Nonetheless, the general view has arisen that just as a chapter (with or without a heading) is a section in the progression of an argument or a story, so within the chapter a paragraph (with or without a subheading) is part of the same orderly progression.
By and large, until the 19th century paragraphs tended to be long and consist of periodic sentences, one period sometimes taking up a paragraph running over one or more pages. In manuals of instruction, however, especially where sections have been logically ordered (and numbered), paragraphs have tended to be shorter. The scripts of prose plays have always had marked-off sections opening with the characters’ names (on a par with verse drama). In novels and other works of fiction, along with the increasing use of separated-off dialogue (similar to the style of scripts), 19th-century writers reduced the lengths of their paragraphs, a process that has continued in the 20th-century, particularly in journalism, advertisements, and publicity materials, where paragraphs are often short and built out of sentence fragments. Writers of fiction often use the same effect to present swift action, changes in thinking, and the like.
Traditionally, teachers of composition have taught students to begin a new paragraph when beginning a new topic or subtopic in an essay or other piece of prose. The aim has been to produce logically ordered sentences, the first of which is a topic or key sentence that sets the scene. This ideal continues to be widely valued, but is not the only basis, or even principal basis, on which paragraphs are constructed by professional writers. In the process of drafting their material, they may combine and recombine paragraphs. Two influences are: relationships with material in preceding and following paragraphs, and the ‘eye appeal’ of different lengths of paragraph arranged in relation to the size of page and the typeface used. Paragraph construction is therefore as much a matter of layout and visual balance as of content and logical relationship between preceding or subsequent paragraphs. For purposes of highlighting or emphasis, longer paragraphs may be divided up, sometimes turning a proposed topic sentence into a topic paragraph. Paragraphs in academic works, works of reference, religious scriptures, specialist journals, consumer magazines, quality newspapers, and tabloid newspapers all follow different rules of thumb in their construction.”
Excerpted from: McArthur, Tom. The Oxford Concise Companion to the English Language. New York: Oxford University Press, 2005.