A comma ( ,
) is a punctuation
mark. It has the same shape as an apostrophe
or single closing quotation mark
in many typefaces, but it differs from them in being placed on the baseline of the text. Some typefaces
render it as a small line, slightly curved or straight, or as a small filled-in number 9.
The comma is used in many contexts and languages, principally for separating things. According to the Oxford English Dictionary, the word comma comes directly from the Greek komma (κόμμα), which means something cut off or a short clause.
In the 3rd century BC, Aristophanes of Byzantium invented a system of single dots (distinctiones) that separated verses (colometry) and indicated the amount of breath needed to complete each fragment of text when reading aloud (not to comply with rules of grammar, which were not applied to punctuation marks until much later). The different lengths were signified by a dot at the bottom, middle, or top of the line. For a short passage (a komma), a media distinctio dot was placed mid-level ( · ). This is the origin of the concept of a comma, though the name came to be used for the mark itself instead of the clause it separated.
The mark used today is descended from a diagonal slash, or virgula suspensiva ( / ), used from the 13th to 17th centuries to represent a pause, notably by Aldus Manutius. In the 16th century, the virgule dropped to the bottom of the line and curved, turning into the shape used today ( , ).
The comma has several uses in English
grammar, all related to marking off separate elements within a sentence:
- Introductory words and phrases: Once upon a time, I didn't know how to use commas.
- Parenthetical phrases: The parenthetical phrase has an important, often misunderstood, use. It is often used for thought interruptions. Information that is unnecessary to the meaning of the sentence is commonly set off and enclosed by commas. If the information is necessary, no commas should be used.
The comma is often used to separate two independent clauses (a group of words that can function as a sentence) that are joined by a coordinating conjunction (for, and, nor, but, or, yet, and so, when they are used to connect; the acronym FANBOYS can be used as a memory aid). Some people feel this is obligatory, while others prefer to use the comma only when not doing so would lead to a different reading.
- Restrictive and non-restrictive use: The sentences I cut down all the trees, which were over six feet tall and I cut down all the trees that were over six feet tall have very different meanings: In the first sentence all the trees were cut down, and a detail (their height) is added. In the second case only some trees were cut down, those over six feet tall. There may have been shorter trees too, but none of them were felled. In the first case, which were over six feet tall is set off by a comma because it is a non-restrictive clause (i.e., its removal does not alter the meaning of the sentence). In the second, that were over six feet tall is a restrictive clause and takes no comma (because if you left it out, the sentence would then say that all the trees were cut down, not just the ones over six feet). Note that in British English, the restrictive clause example can be written as I cut down all the trees which were over six feet tall as the relative pronouns which, who, and whom are interchangeable with that.
- Parenthetical phrases in sentences may include the following:
- Address: My father ate the bagel, John.
- Interjection: My father ate the bagel, gosh darn it!
- Aside: My father, if you don’t mind my telling you this, ate the bagel.
- Appositive: My father, a jaded and bitter man, ate the bagel.
- Absolute phrase: My father, his eyes flashing with rage, ate the bagel.
- Free modifier: My father ate the bagel, chewing with unbridled fury.
- Resumptive modifier: My father ate the bagel, a bagel which no man had yet chewed.
- Summative modifier: My father ate the bagel, a feat which no man had attempted.
- Any phrase that interrupts the flow of the main clause:
- My father, chewing with unbridled fury, ate the bagel (free modifier).
- My father, in a fit of rage, ate the bagel (prepositional phrase).
- My father, with no regard for his health, ate the bagel (adverbial phrase).
- My father, despite his lack of teeth, ate the bagel (adverbial phrase).
- Years following dates (this is American usage—whether this is really parenthetical is moot): My father ate a bagel on December 7, 1941, and never ate one again. (See #10 below.)
- States following cities: My father ate a bagel in Dallas, Texas, in 1963.
- In each case, the parenthesised (as if in parentheses) text is both preceded and followed by a comma, unless that would result in doubling a punctuation mark, or if the parenthetical is at the start or end of the sentence.
The comma is often used to separate a dependent clause from the independent clause if the dependent clause comes first.
- I passed the test, but he failed. (comma) — I passed the test and He failed can function as separate sentences
- I walked home and left shortly after. (no comma) — Although I walked home is independent, left shortly after is dependent on the first part of the sentence
The comma is used to separate co-ordinate adjectives; that is, adjectives that directly and equally modify the following noun. Two questions can be asked to identify adjectives as coordinate adjectives:
- After I brushed the cat, I lint-rollered my clothes. (optional comma)
- I lint-rollered my clothes after I brushed the cat. (no comma)
- Would the meaning be the same if their order were reversed?
- Would the meaning be the same if and were placed between them?
- A positive answer to either of these questions is evidence that a comma should be placed between the adjectives:
The comma is used to separate items in lists.
- In the dull, incessant droning but not the cute little cottage.
- The devious lazy red frog suggests there are lazy red frogs (one of which is devious), while the devious, lazy red frog does not carry this connotation.
A comma is used to set off quoted material that is the grammatical object of an active verb of speaking or writing.
- However, if any of the individual items in the list is complex and long, or contains a comma itself, it is best to use only semicolons (;) to separate the items, and possibly to introduce the list with a colon (:):
- We had soup of the day; then sole meunière, interestingly prepared with an unusual variety of parsley and with lime juice instead of lemon; greens; a fruit salad; and a good port to finish off.
- There were several tasks facing them: shaping the mast, for which they could use an adze or, with some difficulty, an axe; raising the finished mast; and caulking the timbers with whatever suitable material could be found.
- A comma before the final and, or, or nor in a list of more than two elements is called the serial comma. It is also known as the Oxford comma or Harvard comma:
- We had milk, biscuits, and cream.
- The alternative names are derived from Oxford University Press and Harvard University Press, both prominent advocates of this style.
- Although the serial comma is not always used, it sometimes avoids ambiguity. In other cases, it may introduce ambiguity.
- I spoke to the boys, Sam and Tom. — The boys refers to Sam and Tom (I spoke to two people).
- I spoke to the boys, Sam, and Tom. — The boys, Sam, and Tom are separate units (I spoke to four or more people).
- ... x, y and z. This suggests either that y and z are in apposition to x, or that x, y, and z are all separate entities.
- ... x, y, and z. This could mean either that y is in apposition to x (and z is a separate entity), or that x, y, and z are all separate entities.
In representing large numbers, English texts use commas separating each group of three digits. This is almost always done for numbers of six or more digits, and optionally for five (or even four) digits. Note, however, that in other-language texts the numerical use of commas and periods may be reversed (periods to group zeros, comma as decimal point). The International System of Units (SI) now recommends using spaces to separate sets of three digits in numbers more than four digits long (and the comma or period is then used only for the decimal place).
- Mr. Kershner says, You should know how to use a comma.
- Quotations that follow and support an assertion should be set off by a colon rather than a comma:
- Wordsworth recalls his childhood existence as precious but as now outside his grasp: Where is it now, the glory and the dream?
- Quotations that are incorporated in ways other than as the object of active verbs of speaking or writing should be punctuated the same as if there were no quotation marks: Mr. Kershner told me that I should know how to use a comma.
Commas are used when writing names that are presented last name first. Two commas are used when writing the date in the following forms:
- 6,345,990.01 (or 6 345 990.01 according to the SI)
- 4123 (not 4,123 nor 4 123)
A comma is written in a geographical reference between the city and the state and again following the state:
- American English: September 11, 2001, was a momentous day.
- British English: Tuesday, 11 September 2001, was a momentous day.
Fowler's Modern English Usage demonstrates an optional use of commas with two sentences differing only by a comma:
- My dog's masseuse lives in New York, NY, most of the year.
- The rule is, of course, not limited to U.S. states and cities: a comma is used to set off any individual elements in names of geographical places or political divisions (countries, provinces, counties): "The plane landed in Kampala, Uganda, that evening."
The comma is used to indicate that a word has been omitted (only if the reader will understand what the comma has replaced):
- The teacher beat the scholar with a whip. A simple description.
- The teacher beat the scholar, with a whip. Expression of outrage.
- An alternative interpretation is that the second example represents a comma used to remove an ambiguity — to clarify that it was the teacher, not the scholar, who had the whip.
- The cat was white; the dog, brown. (The comma clearly replaces "was.")
Differences between American and British usage
The comma and the quotation mark
pairing can be used in several ways. In American English
, the comma (like most other punctuation marks) is included inside a quotation, no matter what the circumstances. For example:
- My mother gave me the nickname "Johnny Boy," which really made me angry.
However, in British English, punctuation is only placed within quotation marks if it is part of what is being quoted or referred to. Thus:
- My mother gave me the nickname "Johnny Boy", which really made me angry.
The use of the serial comma is sometimes perceived as an Americanism, but in fact varies in both American and British English.
Barbara Child claims that in American English there is a trend toward a decreased use of the comma (Child, 1992, p. 398). This is reinforced by an article by Robert J. Samuelson in Newsweek. Lynne Truss says that this is equally true in the UK and has been a slow, steady trend for at least a century:
Nowadays… A passage peppered with commas — which in the past would have indicated painstaking and authoritative editorial attention — smacks simply of no backbone. People who put in all the commas betray themselves as moral weaklings with empty lives and out-of-date reference books. (Truss, 2004, p. 97–98)
In his 1963 book Of Spies and Stratagems, Stanley P. Lovell recalls that during the Second World War, the British carried the comma over into abbreviations. Specifically, Special Operations, Executive was written S.O.,E. Nowadays, even the full stops are usually discarded.
In many European languages, commas are used as decimal separators. The only English-speaking country which uses this convention is South Africa. Thus, 1,5 V
means one and one-half volts.
Another method of writing numbers is the international system writing style. They write the number fifteen million as 15 000 000. The only punctuation mark is the decimal mark; a period in English text, a comma in all other European languages. Using this convention, twelve thousand fifty-one dollars, seven cents, and 5 mills, would be written in symbols as $12 051.075 in English text, but $12 051,075 in text of any other European language.
In many places, English writers often put commas between each group of three digits. They would write the number fifteen million as 15,000,000. A number with a decimal does not use commas in the fractional portion. Thus, twelve thousand fifty-one dollars, seven cents, 5 mills is written in symbols as $12,051.075.
Many European languages use the exact opposite convention, and the above quantities are written as 15.000.000 and $12.051,075.
In the common character
encoding systems Unicode
, character 44 (0x002C
) corresponds to the comma symbol.
In many computer languages commas are used to separate arguments to a function and to separate elements of a list.
In the C programming language, the comma symbol is an operator which evaluates its first argument (which may have side-effects) and then returns the value of its evaluated second argument. This is useful in for statements and macros.
Diacritical usage (comma below)
The comma is used as a diacritic
mark in Romanian
under the s
), and under the t
). A cedilla
is occasionally used instead of it (notably in the Unicode
glyph names), but this is technically incorrect. A consonant, ḑ
/ Ḑ (d with comma) was used in the Romanian transitional alpabet
(XIXth century) to indicate the sound z
where it was derived from cyrillic ѕ
(/dz/). Ironically, both the comma and cedilla were independent derivatives of a small cursive z
(ʒ) placed below the letter. Thus, in the case of Romanian, ș and ț are in origin shorthand for sz
Comparatively, some consider the diacritics on the Latvian consonants ģ, ķ, ļ, ņ, and formerly ŗ to be cedillas. Although their Adobe glyph names are commas, their names in the Unicode Standard are g, k, l, n, and r with cedilla. They were introduced to the Unicode standard before 1992, and their name cannot be altered.
In the Czech and Slovak languages, the diacritics in the characters ď, ť, and ľ resemble superscript commas, but they are modified carons.
- Barbara Child, Drafting Legal Documents, 2nd Edition, 1992.
- Lynne Truss, Eats, Shoots and Leaves, Gotham Books (2004), ISBN 1-59240-087-6.