Most difficulties with the use of the comma hinge on the distinction between restrictive and non-restrictive sentence elements. A restrictive word, phrase or clause adds to the words it modifies a "restrictive" or defining element that is essential to the meaning of the whole; it should therefore not be separated by a comma or other mark of punctuation. A non-restrictive element provides incidental or supplementary information which does not affect the essential meaning; it should be set off by a comma or commas.
(a) Introductory elements
There are exceptions to the general rule for punctuating restrictive and non-restrictive elements. An introductory phrase or clause, especially if it is a long one, is often followed by a comma even if it is restrictive:
Each of the above sentences could have been correctly punctuated with or without the comma. But an introductory subordinate clause is normally followed by a comma:
After introductory adverbs and short phrases indicating time, frequency, location or cause, the comma is omitted unless needed to avoid ambiguity or add emphasis:
Introductory adverbs or phrases used to mark transition or to express a personal comment are usually set off by commas:
The introductory phrase may also consist of an adjective or participle separated from its noun by the definite or indefinite article:
Conversely, it is sometimes possible to omit the commas that ordinarily set off non-restrictive elements, without obscuring the meaning. This is especially true of short adverbial expressions:
In such sentences the addition of commas not strictly needed for clarity gives emphasis to the elements thus enclosed:
(b) Absolute expressions
One form of non-restrictive expression is the absolute1 construction: a participial phrase grammatically unconnected with the rest of the sentence. Such phrases are followed by a comma:
Note the following errors in the punctuation of absolute expressions:
This common problem is avoided if the sentence is recast so that the subject of both clauses is the same:
(c) Parenthetic expressions
Parenthetic expressions are non-restrictive and therefore require commas:
If a parenthetic expression is removed from the sentence, the remainder of the sentence should read as a coherent, grammatically correct whole. For example, the sentence
is unacceptable because "as good . . . than" is incorrect English. The sentence should be recast as follows:
Occasionally it may be expedient to omit the first of the pair of commas around a parenthetic expression:
The parenthetic phrase here is "without realizing it."
Both commas can sometimes safely be omitted; under no circumstances, however, should the second comma be omitted while the first is retained:
Parenthetic expressions may be set off by parentheses or dashes instead of commas, depending on the degree of emphasis or pause desired, or the length of the expression. Compare:
A common error occurs with parenthetic phrases following the conjunction that. The comma that belongs after the conjunction is often placed before it instead:
Restrictive and non-restrictive appositives should be carefully distinguished. The latter are set off by commas, whereas the former are not:
As in the case of parenthetic expressions, the comma following a non-restrictive appositive cannot be omitted. Thus the sentence
is incorrect. A comma is required after "Gray."
Non-restrictive appositives in final position are usually preceded by a comma:
Often, however, the comma is replaced by a colon or dash:
If the appositive contains internal commas, it is best introduced by a mark other than the comma. In the following example, a colon would be an improvement over the comma after legacy:
(e) Annunciatory expressions
The annunciatory expressions namely, that is and for example are usually followed by a comma. They may be preceded by a comma, a dash, a semicolon or a period, or, together with the matter they introduce, may be enclosed in parentheses, depending on the emphasis desired:
The abbreviations i.e. and e.g.—although these are identical in meaning to that is and for example—should be preceded by a comma, a dash or an opening parenthesis, but need not be followed by a comma.
Note that the expression such as is used to introduce an example, not an appositive, and therefore is not followed by a comma. It may be preceded by a comma or other punctuation, as required in the sentence.
(f) Vocative forms
Vocative forms are non-restrictive and are set off by commas:
Similarly, exclamations and interjections are set off by commas (or exclamation marks):
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