This post outlines the functions of punctuation marks employed at the end of a sentence: the period, the exclamation point, the question mark, and ellipses.
Periods are employed as terminal punctuation for statements other than questions or exclamations. In American English, periods precede a close quotation mark at the end of a sentence (with some technical exceptions in such fields as botany, linguistics, and philosophy). Periods also follow numbers and letters that precede each item in a vertical list.
When an abbreviation ending in a period closes a sentence (such as in “Such abbreviations are common in content pertaining to mathematics, science, etc.”), it does double duty as terminal punctuation; do not add a period. An exclamation point or question mark can follow such use of a period, but revision to avoid consecutive punctuation is advised.
See this post for information about the use of periods in abbreviation.
In formal writing, use of the exclamation point is rare, but it performs a useful function in expressing exclamation of surprise (“That’s absurd!”) or communicating an imperative (“Halt!”). It may also be employed to indicate enthusiasm (“Hi!”).
An exclamation point should replace, not accompany, a comma (“No!” she replied”), though an exception is made when the exclamation is part of the title of a composition or of a component of one (“Her latest painting, titled simply Yes!, is on display”; “The final chapter, ‘Where Do I Go from Here?,’ is essential reading”).
When both an exclamation point and a question mark are appropriate, choose one or the other, though in informal writing, an interrobang, a hybrid of both symbols, can be employed. Frequent use of the exclamation point, or use of two or more in succession, is distracting and should be employed only, for example, to signal in fiction writing the exuberance of a character. An exclamation point in parentheses indicates an editorial interpolation expressing alarm or surprise, as in “A speaker who seriously proposed summary execution (!) was heckled.”
Writers should take care to place an exclamation point before or after a close quotation mark depending on its function. Compare, for example, “John screamed, ‘Get out!’” and “You can believe I was shocked when Mary quietly responded, ‘I know the truth, because I was there’!” In the first sentence, the exclamation point, positioned inside the quotation marks containing John’s outburst, emphasizes the screamed command; in the second sentence, the exclamation point, located outside the quotation marks framing Mary’s reported comment but within those bracketing the reporter’s statement, signals the surprise the reporter felt about Mary’s unexpected but quietly uttered admission.
Exclamation points that are integral to a proper name (for example, in the company name Yahoo! or in the title of the television program Jeopardy!) are usually retained, though they may, especially in the former example, invite confusion. (Ambiguity is unlikely in the case of an exclamation point that is part of a word or phrase formatted in italics or boldface.)
A question mark is employed in place of a period to indicate an interrogative word, phrase, or full sentence—usually the latter, although it may follow a single word or a phrase functioning as a sentence, or one or more interrogative elements can be embedded in a sentence, as in “Was he feeling envy? resentment? humiliation?” (Alternatively, the last two words might be treated as one-word sentences: “Was he feeling envy? Resentment? Humiliation?”)
Question marks should not punctuate indirect questions (“The question is whether the initiative should be funded by taxpayers”), sentences ending with interrogative words (“Naturally, you might ask why”), or formal requests (“Would you please respond at your earliest inconvenience”).
A question mark may also replace or accompany an unknown quantity, as in “John Smith (1452?–1506) . . .” or “John Smith (?–1506) . . . .”
See also the discussion of exclamation points above; all the guidance after the first paragraph in that section applies to question marks as well.
When ellipses end an unfinished sentence, the implication is that the reader is familiar with the full sentence (“When in Rome . . .”), which is delivered in an offhand manner, or that the speaker is faltering (“I was just trying to . . .”). (To represent interrupted speech, use a dash rather than ellipses; see this post about the use of dashes as internal punctuation.)
When representing omission of one or more words at the beginning of a sentence that follows a full sentence, use a period and ellipses as shown here: “Finish each day and be done with it. . . . Tomorrow is a new day.” When indicating elision of one or more words at the end with a complete sentence, which is followed by another sentence, place the period for the first sentence after the ellipses as shown here: “I want to live and feel all the shades, tones and variations of mental and physical experience . . . . And I am horribly limited.” (The period is the fourth dot.) Do not place ellipses at the end of a quotation to indicate that more text follows the quotation in the source material.
The use of ellipses as internal punctuation is discussed in this post.
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Original post: A Guide to Terminal Punctuation