The period, or full stop, marks the end of a declarative sentence. As a sign it has several other uses which will appear in the paragraphs following.
Rules for the Use of the Period
1. At the end of every sentence unless interrogative or exclamatory.
2. After abbreviations.
Nicknames, Sam, Tom, etc., are not regarded as abbreviations.
The metric symbols are treated as abbreviations but the chemical symbols are not. M. (metre) and mg. (milligram) but Na Cl or CO
Per cent is not regarded as an abbreviation.
The names of book sizes (12mo 16mo) are not regarded as abbreviations.
4. The period is now generally omitted in display matter after:
Box heads in tables,
Signatures at the end of letters.
5. The period is omitted:
After Roman numerals, even though they have the value of ordinals.
After MS and similar symbols.
In technical matter, after the recognized abbreviations for linguistic epochs. IE (Indo-European), MHG (Middle High German)
After titles of well-known publications indicated by initials such as AAAPS (Annals of the American Academy of Political Science).
6. When a parenthesis forms the end of a declarative sentence the period is placed outside the parenthesis, as in the preceding example. A period is placed inside a parenthesis only in two cases.
i. After an abbreviation.
This was 50 years ago (i.e. 1860 A.D.)
ii. At the end of an independent sentence lying entirely within the parenthesis.
Lincoln was at the height of his powers in 1860 (He was elected to the presidency at this time.)
7. When a sentence ends with a quotation, the period always goes inside the quotation marks.
I have just read DeVinne's "Practice of Typography."
The same rule applies to the use of the other low marks, comma, semicolon, and colon, in connection with quotation marks. Unlike most rules of grammar and punctuation, this rule does not rest on a logical
basis. It rests on purely typographic considerations, as the arrangement of points indicated by the rule gives a better looking line than can be secured by any other arrangement.
Other Uses of the Period
1. The period is used as a decimal point.
2. The period is used in groups, separated by spaces, to indicate an ellipsis.
He read as follows: "The gentleman said . . . he was there and saw . . . the act in question."
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