The comma is the most frequently misused punctuation mark, and many of the rules governing its use are vague and riddled with exceptions. The writer must frequently rely on personal judgment and should be guided by considerations of clarity more than by any particular set of rules.
Note that, as a general rule, commas interrupt the flow of a sentence and should therefore not be used where they do not contribute to clarity. A sentence requiring a large number of commas for clarity is probably a poorly constructed one in need of rephrasing. Yet the comma is also the mark most often incorrectly omitted.
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):
Elements of equal rank or relation in a sentence are said to be co-ordinate. The co-ordinate elements may be words or phrases in a series, or they may be entire clauses.
(a) Nouns and noun phrases
Items in a series may be separated by commas:
They may also be linked by co-ordinating conjunctions such as and or or:
Opinions differ on whether and when a comma should be inserted before the final and or or in a sequence. In keeping with the general trend toward less punctuation, the final comma is best omitted where clarity permits, unless there is a need to emphasize the last element in the series. This comma is usually omitted in the names of firms and always before an ampersand:
On the other hand, it is usually inserted if the items in the series are phrases or clauses of some length, or if omission of the comma might lead to ambiguity or misunderstanding:
A comma is also required before etc.:
A more complex situation occurs when apposition commas are used together with co-ordinating commas, as illustrated below:
The sentence should be rephrased so that no non-restrictive appositive occurs within a co-ordinate element:
Alternatively, semicolons may be used to separate elements in a complex series (see also 7.23 The Semicolon, Co-ordinate elements):
A comma is normally used to separate two main clauses in a compound sentence when they are joined by a co-ordinating conjunction (and, but, or, nor, yet or for):
If the clauses are short or closely related, the commas may be omitted before and, but, or or nor:
Co-ordinate clauses not joined by a co-ordinating conjunction are usually separated by a heavier mark of punctuation than the comma:
A comma will suffice, however, if the clauses are short, or if the writer wishes to emphasize a contrast or lead the reader on to the following clause as quickly as possible:
When a number of independent co-ordinate clauses follow one another, a comma should be used after each one except (usually) the last, in accordance with the rule for items in a series (see 7.15 The Comma, Co-ordinate elements(a)):
It is a common error to confuse a simple sentence having a compound predicate with a compound sentence requiring a comma between clauses. Note the difference between the following examples:
Where the clauses of a compound sentence are joined by a conjunctive adverb (such as however, instead, meanwhile, otherwise, similarly, so, still, then, therefore or yet), a semicolon is usually called for, though a comma will often suffice before so, then and yet:
A series of adjectives modifying a noun may or may not be co-ordinate. The adjectives are co-ordinate if their order does not affect the meaning, in which case they should be separated by a comma. If they are not co-ordinate, that is, if one adjective modifies the phrase formed by the following adjective(s) plus the noun, then they should not be separated by a comma:
Adjectives of both types may of course occur together:
The rule stated above, however, is not an infallible guide. When in doubt omit the comma, as in:
The final adjective in the series should not be separated from the following noun by a comma:
(d) Antithetic expressions
Antithetic expressions are usually separated by a comma:
However, short expressions of this type may not require a comma:
Sometimes the reader will be led astray by a word or phrase which appears at first to be used in one sense but turns out from the context to be used in another. In all the following examples, commas should have been used in order to prevent misreading:
The comma can be a useful device for securing a pause or emphasis:
A comma may be used to indicate that words have been omitted:
Again, the comma may be omitted if clarity is not compromised.
Place a comma after words introducing short direct quotations, declarations and direct questions (a colon is needed to introduce longer sentences):
Note the capital letter and the absence of quotation marks in the last two examples.
If the quotation or question follows a form of the verb to be, is in apposition to a noun, or is worked naturally into the syntax of the sentence, no comma is needed:
It is also acceptable to omit the comma before quotations introduced by verbs of saying:
The use of punctuation in quotations is discussed in 8.03 Punctuation and grammar in run-in quotations.
Commas are used around titles and degrees within the body of a sentence:
A comma is placed between a surname and a given name or initials if the surname is written first:
Chinese and Vietnamese names are an exception. They are written with the family name first and no comma:
Use a comma to separate the day of the week from the date and the place from the date:
If the date is written in the order day-month-year, no commas are required before, after or between the components of the date:
If, however, the order given is month-day-year, the day and year are separated by a comma, and the year should normally be followed by a comma within the body of a sentence or sentence equivalent:
If you are stating only the month and the year, do not insert a comma:
Similarly, a comma separates a place name from the name of a province or the abbreviation for that province, and the province’s name or abbreviation is normally followed by a comma within the body of a sentence or sentence equivalent:
Use commas to separate address components, as illustrated:
Note that the postal code is followed, but not preceded, by a comma when the address forms part of a sentence, and that two spaces separate the provincial name from the postal code.
Do not use commas between the name and the number of an organizational unit:
Do not insert commas in numerical expressions such as the following:
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