The Oxford comma (also known as the serial 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 Oxford 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 Oxford 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 Oxford 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 Oxford comma by default include:
4. that it better matches the spoken cadence of sentences;
5. that it sometimes reduces ambiguity;
6. 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 Oxford comma by default include:
4. that it is against conventional practice;
5. that it may introduce ambiguity (see examples below); and
6. 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 Oxford comma, and make recommendations in a more nuanced way (see Usage and subsequent sections).
Use of the Oxford 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.
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 Oxford comma removes this ambiguity. With a comma after peanut butter, the kinds of sandwich are these:
7. cream cheese and peanut butter
With a comma after cream cheese, the kinds of sandwich are these:
7. cream cheese
8. peanut butter and jelly
Some writers who normally avoid the Oxford comma may use one in these circumstances, though sometimes re-ordering the elements of such a list can help as well.
Use of the Oxford 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 Oxford 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.
Betty, a maid and a rabbit.
When the Oxford 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.
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 Oxford 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 Oxford 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 Oxford 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)
• 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.
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 Oxford 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 Oxford 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 Oxford 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 Oxford comma:
The United States Government Printing Office's Style Manual
After each member within a series of three or more words,