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Use commas sparingly, only when breaks are needed for readability and to avoid confusion or misinterpretation. APS style requires the use of the series comma. When in doubt, follow the guidelines in CBE, 6th ed., and The Chicago Manual of Style.

When to use a comma:

To separate the elements (words, phrases, or clauses) of a simple series of more than two. If any of the elements contain internal puctuation, separate them with semicolons.

To separate two independent clauses joined by a coordinating conjunction. CBE makes the comma optional if the clauses are not long or complex. Follow the author's lead in a manuscript unless the comma is needed for clarity.

To set off an introductory clause beginning with a subordinating conjunction (if, although, because, when, since). CBE makes it optional if the clause is short; however, that is unusual in scientific writing. If the subordinate clause falls at the end of the sentence, follow the manuscript for use of the comma before the subordinating conjuction.

To set off an introductory infinitive phrase ("To accomplish the task, we...") or participial phrase ("Using hippocampal slices from adult rats, we studied...") Beware of dangling participles; it is a mistake that authors make frequently! In the previous example, you would not change "using" to "with" or "with the use of."

To set off a single introductory prepositional phrase to prevent misreading. A comma after a short introductory prepositional phrase is optional. Always insert a comma after a series of two or more introductory prepositional phrases.

In all, eight experiments were performed.

After three periods of eight hours each, the rats were removed to...

To separate a nonrestrictive phrase or clause from the rest of the sentence. A nonrestrictive clause begins with "which." Frequently authors use "which" for both restrictive and nonrestrictive clauses; change "which" to "that" or vice versa when required.


Rats were fasted for 4 h before each experiment, which was sufficient time to establish a steady state.

The animals, which were fasted overnight, were then injected with 50 ml of...


The high prevalence of antibodies to the Bartonella species which were examined in this study indicates a possibility of infection...

To separate a nonrestrictive appositive from the rest of the sentence.

Raymond Turner, a biologist, described the species.

To set off a conjunctive adverb (therefore, thus, still, however, accordingly, moreover, nevertheless, consequently) or a transitional phrase (by comparison, on the other hand, in fact) that introduces a distinct break in the continuity of thought (an interjected or parenthetical phrase). If the author has not used commas, do not insert.

To set off "i.e." ("that is") and "e.g." ("for example"), whether abbreviated or expanded; to set off "respectively", which usually falls at the end of the sentence.

Before injection of picrotoxin, the cell responded in its normal way, i.e., most strongly to ipsilateral, small-field motion from front to back.

The longest delays included in the calculations were 14, 9, and 4 ms for the FM-FM, DF, and VF areas, respectively.

After multiple adjectives modifying the same noun.

monosynaptic, excitatory linkage

binocular, optokinetic stimulation

Inside double quotation marks, even when the comma is not part of the quoted material.

Inside superior letters or numbers used as footnotes.

To set off contrasted or antithetical phrases or clauses.

The greater the risks, the more gratifying will be the results.

To set off words, phrases, or clauses placed out of their natural order for emphasis or clarity.

To set off adjacent numbers.

In 1908, 45 experiments were...

When not to use a comma:

Before the word "and" when it connects two parallel elements, e.g., subjects, verbs, phrases, or dependent clauses. This is a frequent comma error in both foreign and US papers.

Dopamine is released from carotid body glomus cells and chemoafferent fibers in the nucleus tractus solitarii.

Before or after a one-word appositive.

The GABAA receptor agonist muscimol did not influence the response of WDR neurons in normal or allodynic animals.

With a restrictive adjective clause or phrase. (A restrictive clause begins with "that," although many authors use "which" for both restrictive and nonrestrictive modifiers.) As a general rule, do not add commas before and after prepositional phrases (making them nonrestrictive).


The occurrence of different mechanisms that depend on the degree of hypoxia could explain these differences.


Results with knockout mice are most clearly interpreted in experimental paradigms, that take place over days to weeks.

To separate coordinate clauses joined by a conjunctive adverb (e.g., "however") rather than a coordinating conjunction. Use a semicolon instead.

It was not possible to apply parametric trend tests to these data; however, a Wilcoxon's test revealed that overall the discharge rate was significantly higher after the injury.

After the abbreviation "cf." ("compare").

After "thus" or "therefore" unless followed by a subsidiary clause or phrase:

Thus activation threshold gave no evidence for major contributions from nonsoma sites.

Thus, when the neurons with set-related activity showed amplitude selectivity, they showed greater activity change during delay 2 before larger movements.

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last edited 07/23/03