Serial comma

The serial comma (also known as the Oxford comma or Harvard comma) is the comma used immediately before a grammatical conjunction (nearly always and or or; sometimes nor) that precedes the last item in a list of three or more items. The phrase "Portugal, Spain, and France", for example, is written with the serial comma, while "Portugal, Spain and France", identical in meaning, is written without it.

There is no global consensus among writers or editors on the use of the serial comma. Most authorities on American English recommend its use, but it is not so frequently used in British English (see extended treatment below, including a survey of published recommendations in Usage and subsequent sections). In many languages (e.g. French, Italian, Polish, Spanish) the serial comma is not normally used, although it may be employed in cases where it aids clarity or the prosody to be used when reading.

Arguments for and against

Arguments typically advanced for use of the serial comma by default include:

that it better matches the spoken cadence of sentences;
that it sometimes reduces ambiguity;
that its use matches practice with other means of separating items in a list (example: when semicolons are used to separate items, a semicolon is consistently included before the last item, even when and or or is present);

Arguments typically advanced for avoidance of the serial comma by default include:

that it is against conventional practice;
that it may introduce ambiguity (see examples below); and
that it is redundant, since the and or the or serves by itself to mark the logical separation between the final two items.

Many sources, however, are against both automatic use and automatic avoidance of the serial comma, making recommendations in a more nuanced way (see Usage and subsequent sections).

Ambiguity

Resolving ambiguity

Use of the serial comma can sometimes remove ambiguity. Consider the possibly apocryphal book dedication quoted by Teresa Nielsen Hayden:

To my parents, Ayn Rand and God.

There is ambiguity about the writer's parentage, because Ayn Rand and God can be read as in apposition to my parents, leading the reader to believe that the writer refers to Ayn Rand and God as his or her parents. A comma before and removes the ambiguity:

To my parents, Ayn Rand, and God.

Consider also:

My favourite types of sandwiches are pastrami, ham, cream cheese and peanut butter and jelly.

According to the two most plausible interpretations of this sentence, four kinds of sandwich are listed. But it is uncertain which are the third and fourth kinds. Adding a serial comma removes this ambiguity. With a comma after peanut butter, the kinds of sandwich are these:

pastrami
ham
cream cheese and peanut butter
jelly

With a comma after cream cheese, the kinds of sandwich are these:

pastrami
ham
cream cheese
peanut butter and jelly

Some writers who normally avoid the serial comma may use one in these circumstances, though sometimes re-ordering the elements of such a list can help as well.

Creating ambiguity

Use of the serial comma can introduce ambiguity. An example would be a book dedication reading:

To my mother, Ayn Rand and God.

In the context of the no-serial-comma convention this is unambiguously a list of three, but introducing a serial comma creates ambiguity about the writer's mother, because "Ayn Rand" can then be read as in apposition to "my mother" (with the commas fulfilling a parenthetical function):

To my mother, Ayn Rand, and God.

This ambiguity could be resolved by restating the preposition before each list item:

To my mother, to Ayn Rand, and to God.

Consider also:

Betty, a maid and a rabbit.

When the serial comma is not used, this is clearly a list of two people and a rabbit (assuming that the unlikely idea that Betty is both a maid and a rabbit is rejected), whereas

Betty, a maid, and a rabbit

may refer either to one person (Betty, who is a maid) or to two people (Betty and a maid) and a rabbit.

Unresolved ambiguity

The Times once published this description of a Peter Ustinov documentary: "highlights of his global tour include encounters with Nelson Mandela, an 800-year-old demigod and a dildo collector. This is ambiguous as it stands, but even if a serial comma were added Mandela could still be mistaken for a demigod.

Or consider "They went to Oregon with Betty, a maid, and a cook." The presence of the last comma in the list creates the possibility that Betty is a maid, reasonably allowing it to be read either as a list of two people or as a list of three people, context aside. On the other hand, removing the comma leaves the possibility that Betty is both a maid and a cook; so in this case neither the use nor the avoidance of the serial comma resolves the ambiguity.

A writer who intends that Betty, the maid, and the cook be taken as three distinct people may create an ambiguous sentence, regardless of whether the serial comma is adopted. Furthermore, if the reader is unaware of which convention is being used, both versions are always ambiguous.

These forms (among others) would remove the ambiguity:

They went to Oregon with Betty – a maid and a cook. (One person)
They went to Oregon with Betty, who is a maid and a cook. (One person)
They went to Oregon with Betty (a maid) and a cook. (Two people)
They went to Oregon with Betty – a maid – and a cook. (Two people)
They went to Oregon with the maid Betty and a cook. (Two people)
They went to Oregon with Betty and a maid and a cook. (Three people)
They went to Oregon with Betty, one maid and a cook. (Three people)
They went to Oregon with a full staff: Betty; a maid; and a cook. (Three people)
They went to Oregon with a maid, a cook, and Betty. (Three people)
They went to Oregon with a maid, a cook and Betty. (Three people)

In general:

The list x, y and z is unambiguous if y and z cannot be read as in apposition to x.
Equally, x, y, and z is unambiguous if y cannot be read as in apposition to x.
If neither y nor y[,] and z can be read as in apposition to x, then both forms of the list are unambiguous; but if y or y[,] and z can be read as in apposition to x, then both forms of the list are ambiguous.
x and y and z is unambiguous.

Usage

The Chicago Manual of Style, Strunk and White's Elements of Style, most authorities on American English and Canadian English, and some authorities on British English (for example, Oxford University Press, and Fowler's Modern English Usage) recommend the use of the serial comma. Newspaper style guides (such as those published by The New York Times, the Associated Press, The Times newspaper in the United Kingdom, and the Canadian Press) recommend against it, possibly for economy of space.

The differences of opinion on the use of the serial comma are well characterized by Lynne Truss in her popularized style guide Eats, Shoots & Leaves: "There are people who embrace the Oxford comma, and people who don't, and I'll just say this, never get between these people when drink has been taken.

In Australia, Canada, South Africa and the United Kingdom, the serial comma tends not to be used in non-academic publications unless its absence produces ambiguity. Many academic publishers (for example, Cambridge University Press) also avoid it, though some academic publishing houses in these countries do use it. The Australian Government Publishing Service's Style Manual for Authors, Editors and Printers (6th edition, 2002) recommends against it, except "to ensure clarity" (p. 102).

Style guides supporting mandatory use

The following style guides support mandatory use of the serial comma:The United States Government Printing Office's Style Manual:

After each member within a series of three or more words, phrases, letters, or figures used with and, or, or nor.

*"red, white, and blue"

*"horses, mules, and cattle; but horses and mules and cattle"

*"by the bolt, by the yard, or in remnants"

*"a, b, and c"

*"neither snow, rain, nor sleet"

*"2 days, 3 hours, and 4 minutes (series); but 70 years 11 months 6 days (age)"Wilson Follett's Modern American Usage: A Guide (Random House, 1981), pp. 397-401:

What, then, are the arguments for omitting the last comma? Only one is cogent – the saving of space. In the narrow width of a newspaper column this saving counts for more than elsewhere, which is why the omission is so nearly universal in journalism. But here or anywhere one must question whether the advantage outweighs the confusion caused by the omission ...

The recommendation here is that [writers] use the comma between all members of a series, including the last two, on the common-sense ground that to do so will preclude ambiguities and annoyances at a negligible cost.

Chicago Manual of Style, 15th edition (University of Chicago Press, 2003), paragraph 6.19:

When a conjunction joins the last two elements in a series, a comma ... should appear before the conjunction. Chicago strongly recommends this widely practiced usage....

*"She took a photograph of her parents, the president, and the vice president."

*"I want no ifs, ands, or buts."

*"The meal consisted of soup, salad, and macaroni and cheese."

Texas Law Review Manual on Usage, Style & Editing (10th ed. 2005), R. 1.16The American Medical Association Manual of Style, 9th edition (1998) Chapter 6.2.1:

Use a comma before the conjunction that precedes the last term in a series.

*Outcomes result from a complex interaction of medical care and genetic, environmental, and behavioral factors.

*The physician, the nurse, and the family could not convince the patient to take his medication daily.

*While in the hospital, these patients required neuroleptics, maximal observation, and seclusion.The Publication Manual of the American Psychological Association, 5th edition (2001) Chapter 3.02:

Use a comma between elements (including before and and or) in a series of three or more items.

*the height, width, or depth

*in a study by Stacy, Newcomb, and BentlerThe Elements of Style (Strunk and White, 4th edition 1999):

In a series of three or more terms with a single conjunction, use a comma after each term except the last.

The Oxford Style Manual, 2002, Chapter 5, section 5.3 Comma:

"For a century it has been part of OUP style to retain or impose this last serial (or series) comma consistently, [...] but it is commonly used by many other publishers both here and abroad, and forms a routine part of style in US and Canadian English. [...] Given that the final comma is sometimes necessary to prevent ambiguity, it is logical to impose it uniformly, so as to obviate the need to pause and gauge each enumeration on the likelihood of its being misunderstood – especially since that likelihood is often more obvious to the reader than the writer." (pp. 121–122)

The CSE Manual for Authors, Editors, and Publishers (Council of Science Editors, 7th edition, 2006), Section 5.3.3.1

To separate the elements (words, phrases, clauses) of a simple series of more than 2 elements, including a comma before the closing “and” or “or” (the so-called serial comma). Routine use of the serial comma helps to prevent ambiguity.

Garner's American Usage (Oxford, 2003)

Whether to include the serial comma has sparked many arguments. But it's easily answered in favor of inclusion because omitting the final comma may cause ambiguities, whereas including it never will.

Most college writing handbooks in the U.S. also advocate use of the serial comma.

Style guides opposing mandatory use

The Times style manual:

Avoid the so-called Oxford comma; say "he ate bread, butter and jam" rather than "he ate bread, butter, and jam".

The New York Times stylebook:

In general, do not use a comma before and or or in a series: The snow stalled cars, buses and trains.

The Economist style manual:

Do not put a comma before and at the end of a sequence of items unless one of the items includes another and. Thus 'The doctor suggested an aspirin, half a grapefruit and a cup of broth. But he ordered scrambled eggs, whisky and soda, and a selection from the trolley.'

The AP Stylebook:

Use commas to separate elements in a series, but do not put a comma before the conjunction in a simple series: The flag is red, white and blue. He would nominate Tom, Dick or Harry.

Put a comma before the concluding conjunction in a series, however, if an integral element of the series requires a conjunction: I had orange juice, toast, and ham and eggs for breakfast.

Use a comma also before the concluding conjunction in a complex series of phrases: The main points to consider are whether the athletes are skillful enough to compete, whether they have the stamina to endure the training, and whether they have the proper mental attitude.

The Australian Government Publishing Service's Style Manual for Authors, Editors and Printers:

A comma is used before and, or, or etc. in a list when its omission might either give rise to ambiguity or cause the last word or phrase to be construed with a preposition in the preceding phrase: "There were many expeditions, including those of Sturt, Mitchell, Burke and Wills, and Darling." "The long days at work, the nights of intense study, and inadequate food eventually caused them serious health problems." "The sea, the perfume of wisteria, or a summer lunch: any of these revived memories of an easier time." "We needed to know how to get there, what time to get there, the number of participants, etc."

Generally, however, a comma is not used before and, or or etc. in a list: "John, Warren and Peter came to dinner." "Fruit, vegetables or cereals may be substituted." "Why not hire your skis, boots, overpants etc.?"

The Guardian Style Guide:

a comma before the final "and" in lists: straightforward ones (he ate ham, eggs and chips) do not need one, but sometimes it can help the reader (he ate cereal, kippers, bacon, eggs, toast and marmalade, and tea)

University of Oxford Writing and Style Guide:

As a general rule, do not use the serial/Oxford comma: so write ‘a, b and c’ not ‘a, b, and c’. But when a comma would assist in the meaning of the sentence or helps to resolve ambiguity, it can be used – especially where one of the items in the list is already joined by ‘and’: