Commas: what a sentence includes, excludes and joins
Commas group and separate words, phrases and clauses. Their position can change which items a statement includes, which information identifies its subject and where one clause ends and another begins.
Main functions
A comma marks a slight break within a sentence and can clarify meaning by grouping or separating its parts. English comma conventions differ from those of other languages, and both native and non-native writers may use too many or too few. Scientific-writing guidance gives these examples of its main functions:
| Function | Example |
|---|---|
| Serial comma | Scientists should exercise great care when drafting, structuring, and proofreading a manuscript. |
| Essential clause | The patients who were eligible were randomly allocated to one of the two study groups. |
| Nonessential clause | The patients, who had been fully informed about the study procedure, were randomly allocated to one of the two study groups. |
| Compound sentence | We used the Wilcoxon rank test, and all data were tabulated. |
| Introductory phrase | After mixing the solvents, we added the test compound. |
| Introductory adverb | However, we only used the viable cells. |
Comma-omission repairs include these five patterns:
| Original pattern | Repair |
|---|---|
| Conjunctive adverb + independent clause | Conjunctive adverb, independent clause |
| Prepositional phrase + independent clause | Prepositional phrase, independent clause |
| Adverbial clause + independent clause | Adverbial clause, independent clause |
| Independent clause and independent clause | Independent clause, and independent clause |
| Noun phrase, noun phrase or noun phrase | Noun phrase, noun phrase, or noun phrase |
Appositives and nonessential information
An appositive stands beside another element, usually a noun phrase, and supplies information about it; it can appear at the beginning, middle or end of a sentence. A restrictive appositive limits who or what is meant, whereas a nonrestrictive appositive can be deleted without changing which person or thing is meant. The distinction is illustrated by Bella's brother Joseph is a nurse, where the name distinguishes Joseph from other brothers, and Bella's brother, Joseph, is a nurse, where Joseph is presented as her only brother.
A nonrestrictive appositive is parenthetical: the sentence remains grammatically complete without it, and commas, parentheses or dashes set it off. Examples include Glucose is a hexose, a 6-carbon sugar, The smallpox virus (an orthopoxvirus) is a double-stranded DNA virus and Nobody—absolutely no one—can be allowed in the hospital nursery without identification.
Adjectives before a noun
Adjective punctuation distinguishes cumulative adjectives from coordinate adjectives. The rules are:
- Do not put a comma after a determiner.
- Do not put a comma between adjectives from different categories: cumulative adjectives.
- Do not put a comma between the last adjective and the noun it modifies.
- Use a comma or and between adjectives from the same category: coordinate adjectives.
A pair is coordinate if the phrase still sounds right and makes sense both with and between the adjectives and with their order reversed. Examples are his cruel, callous behavior, cold, rainy weather and cold and clammy hands.
No comma separates adjectives when one modifies the whole idea expressed by the next adjective and noun, as in a good old friend or a clever young man. Thus, healthy adult subjects means adult subjects who are healthy and takes no comma between healthy and adult; the same applies to healthy teenage subjects or healthy adolescent subjects.
For the longer phrase a prospective two-armed randomized controlled clinical trial, one usage adviser recommended a prospective, two-armed, randomized, controlled clinical trial to ease reading, with no comma within clinical trial.
A comma can be grammatically acceptable yet distracting. In the soluble, truncated form of HA, both adjectives modify form and could be joined with and or reversed, but an editing recommendation removes the comma to avoid separately emphasising soluble or briefly suggesting two forms. In that reading, truncated describes the form more innately than soluble, which may be a consequence of truncation.
Introductory words, phrases and clauses
A comma after an introductory qualification or link marks its boundary and can prevent a false first reading. Although rare infection can occur initially suggests a rare infection; Although rare, infection can occur instead marks although rare as the qualification. However unlike the previous type qualifies the degree of unlikeness, whereas However, unlike the previous type introduces a contrast with the preceding text.
A conjunctive adverb is similarly separated: Therefore, it was important to obtain normative test data for adolescents uses the comma to mark and emphasise the link to the previous sentence. In For the patient group, data were recorded from review (chart and x-ray), questionnaire, and neurologic tests, the comma prevents data being read as part of for the patient group data. The introductory position also distances one modifier from the verb and avoids modifier clutter.
An adverbial clause before the independent clause is conventionally followed by a comma: If the data had not been analysed for heterogeneity, the results would have been the same as those of previous studies. A following subordinate clause generally has no preceding comma, including clauses introduced by because, since, if or that. A following contrast clause beginning whereas is often separated; one editing view treats commas before following clauses of reason, condition or purpose as justified, if at all, by emphasis.
Independent clauses and coordinated parts
A comma before a conjunction can prevent the second subject being mistaken for another object in the first clause. I kissed Joan, and Mary laughed avoids initially reading Joan and Mary as a pair of objects. Likewise, A lipid fraction was incubated with 6% KOH, and released fatty acids were extracted with hexane marks released fatty acids as the second subject rather than another object of with. One editing recommendation is to use this comma consistently between independent clauses joined by and, even where confusion is unlikely.
Repeating an omitted auxiliary can reinforce the boundary: All animals were anaesthetised with pentobarbitone, and blood samples were taken from an abdominal blood vessel makes the second clause explicit. The longer and more complex the coordinated clauses, the more useful a boundary comma becomes. Where house style permits, close punctuation uses all the marks suggested by the grammatical structure, even though some readers find the extra commas choppy.
Comma-overuse repairs distinguish coordinated subjects, parallel verbs and non-parallel verbs:
| Original pattern | Revision | Action |
|---|---|---|
| Subject, and subject | Subject and subject | Delete comma |
| Subject verb A, and parallel verb B | Subject verb A and parallel verb B | Delete comma |
| Subject verb A, and non-parallel verb B | Subject participial phrase verb B | Convert verb A into a participial phrase |
| Subject, participial phrase, verb B | Add paired commas | |
| Participial phrase, subject verb B | Move the participial phrase to the start and follow it with a comma |
A useful test removes modifiers to expose the skeleton: Actin and Sam were purchased takes no comma between its subjects, and adding the antibody descriptions and catalogue numbers does not create one. Fertilised hen eggs were routinely incubated and staged likewise takes no comma before and; no extra mark is needed to indicate sequence in this example. He has several years' experience and is thoroughly competent is another example where usage advice omits the comma.
Non-parallel predicates can be recast rather than separated by a comma. An editing example replaces The Introduction section is no exception, and would be improved by the usage of subheadings with three alternatives:
- The Introduction being no exception would be improved by the usage of subheadings.
- The Introduction, being no exception, would be improved by the usage of subheadings.
- Being no exception, the Introduction would be improved by the usage of subheadings.
The first leaves emphasis on would be improved; the second emphasises the participial phrase but interrupts the subject-to-verb path; the third supplies context at the start without that interruption.
Unnecessary commas can also disrupt parallel dependent noun clauses by suggesting a contrast. In It is possible that these two types of tumours originate from common pax3- and pax-7-expressing cells, and that subsequent genetic alterations drive the cells into different outcomes, removing the comma and numbering the clauses as (1) that … and (2) that … makes their parallel relationship explicit.
Serial commas
The serial or Oxford comma appears before the final conjunction in a list. Advice differs: one usage adviser treats it as optional and follows house style, while other scientific-writing guidance recommends routinely including it to prevent misreading.
My parents, Albert Einstein, and Madame Curie keeps the parents separate from the two named people; without the second comma, the names can be read as identifying the parents. In prostaglandin E, thromboxane A, or 6-keto PGF, the final comma reduces the chance that the last term is read as another name for the one before it. One recommendation is to use serial commas throughout an article if even one list needs them for clarity.
A serial comma can also leave ambiguity: robbery, aggravated and simple assault, recklessly endangering others, a weapons offense, and conspiracy can be read as four charges with a parenthetical gloss or as five separate charges. Parentheses or semicolons can make that distinction explicit. Conversely, Please state name, age, sex and housing requirements illustrates a case where a serial comma helps separate the final items.
Commas around a shared phrase
In a 2008 device-label punctuation example, sale by or on the order of a physician was clarified as sale by, or on the order of, a physician, or expanded to sale by a physician or on the order of a physician. The usage adviser nevertheless considered sale by ambiguous between buying and selling, and reported that the question remained unresolved after seeking a regulatory answer.
Commas after e.g. and i.e.
A 2012 comparison of style guidance reported that Strunk and White required a comma after e.g. and i.e., while the European Commission's translation style guide and the Oxford Guide to Style omitted it. The Commission's guide called for a comma, colon or dash before the abbreviations, and the Oxford guide noted the following comma's use in US practice. The AMA Manual of Style also used a following comma. The argument for omitting the following comma treats the whole phrase as parenthetical: Any OTC pain medication, e.g. paracetamol, should be recorded parallels Any OTC pain medication (e.g. paracetamol) should be recorded.
Direct address
Let me call you sweetheart differs from Let me call you, sweetheart: the comma makes sweetheart an address to the listener rather than the name the speaker proposes to use.