The purpose of the paragraph is to aid the reader to comprehend the thought to be expressed. The paragraph groups in a logical way the different ideas to be communicated. It gives rest to the eye of the reader, and makes clearer the fact that there is a change of topic at each new paragraph.
142. PARAGRAPH LENGTH. There is no fixed rule governing the proper length of the paragraph, but, probably, no paragraph need be more than three hundred words in length. If the whole composition is not more than two hundred and fifty words in length, it will not often need to be subdivided into paragraphs. In a letter, paragraphing should be more frequent than in other compositions.
Paragraphing should not be too frequent. If paragraphing is too frequent, by making each minute subdivision of equal importance, it defeats its purpose of grouping ideas about some general topic.
143. Sometimes a sentence or even a part of a sentence may be set off as a separate paragraph in order to secure greater emphasis. This, however, is only using the paragraph for a proper purpose--to aid in gaining clearness.
144. PARAGRAPHING OF SPEECH. In a narrative, each direct quotation, together with the rest of the sentence of which it is a part, should constitute a separate paragraph. This rule should be always followed in writing a conversation. Examine the following:
A certain Scotch family cherishes this anecdote of a trip which Dr. Samuel Johnson made to Scotland. He had stopped at the house of this family for a meal, and was helped to the national dish. During the meal the hostess asked:
"Dr. Johnson, what do you think of our Scotch broth?"
"Madam," was the answer, "in my opinion it is fit only for pigs."
The only case in which the quoted words can be detached from the remainder of the sentence is where they form the end of the sentence after some introductory words, as in the second paragraph of the example just given.
145. INDENTATION OF THE PARAGRAPH. The first sentence of each new paragraph should be indented. See example under §144. No other sentence should be so indented.
146. The essential qualities which each paragraph should have are: Unity, Coherence, and Emphasis.
UNITY. Unity requires that the paragraph should deal with only one subject, and should include nothing which does not have a direct bearing on that subject. Thus, in the following paragraph, the italicized sentence violates the principle of Unity, because, very obviously it belongs to some other paragraph: