Commas with single words(1) Adjectives before nouns are separated by commas unless there is a strong association between a noun and the adjective immediately preceding it: compare an enterprising, ambitious person and a distinguished foreign politician. When a conjunction such as and or but separates the adjectives, a comma is not used, but a pair of commas suggests an aside or gives emphasis: compare a great and generous leader and a great, and generous, leader (with spoken emphasis on generous). (2) With nouns, adverbs, and adjectives more generally, commas are normally used only with three or more words in sequence: They spoke slowly, deliberately, and softly; We found guns, ammunition, and grenades; The children were happy, noisy, and overexcited. (For the comma before and, see serial comma, below.) A comma marking off two such words conveys emphasis, as would a dash: They spoke slowly, and deliberately. (3) The same considerations apply to a sequence of verbs (They arrived, shouted, and ran in) and prepositions (You will find it in, on, and under everything). (4) However and therefore are often placed between parenthetical commas: I do not, however, want you to go; We decided, therefore, to leave (compare We therefore decided to leave). Parenthetical commas are optional with moreover, nevertheless, unfortunately, etc.
Commas with phrases(1) Phrases such as far and wide, by and large are treated like single words: They travelled far and wide through Spain; She was, by and large, very disappointed in the result. (2) Parenthetical commas may be used to highlight a phrase in much the same way as dashes and brackets, but form a less distinct break: The witness, a middle-aged woman, then stepped forward; Their new song, ‘Singing through my tears’, was published this year. (3) Commas are used to separate items in a list or sequence, as in cases already given. Usage varies as to the inclusion of a comma before and in the last item (bring a chair, a bottle of wine, and a good book). This practice is controversial and is known as the serial comma or Oxford comma, because it is part of the house style of Oxford University Press. It is often superfluous, and there are occasions when the sense requires it to be omitted, but on many occasions it serves to avoid ambiguity: These colours are available: red, green, yellow, and black and white as opposed to red, green, yellow and black and white. (4) Practice varies regarding introductory phrases. A comma is often but not always helpful, and the important point is to achieve a consistent and clear approach: In the meantime, we must wait; In the circumstances, I will agree to your request; In 1939, he joined the army. When there is an implied verb, especially be, a comma is usual (Although ill, she managed to finish the work; Whatever the difficulties, we must keep trying).
Commas with clausesCommas are used in longer stretches of writing in the following ways:(1) To separate two main clauses when emphasis or contrast is needed: compare He woke up and immediately got out of bed and He woke up, and immediately got out of bed: in the second example, the emphasis of sense is on the second clause, whereas in the first there are two more or less equal statements.(2) To mark off the main clause of a longer compound sentence when the two parts are not close enough in meaning or content to form a continuous statement and are not distinct enough to warrant a semicolon. A conjunction such as and, but, yet, etc., is normally used: The road runs through a beautiful wooded valley, and the railway line follows it closely. It is usually considered incorrect to omit the conjunction: I like swimming very much, I go to the pool every week conventionally requires either the insertion of and before the second I, or the use of a colon or semicolon instead of the comma. However, there is no absolute rule: many writers place two short sentences together in this way (creating what is called a comma splice) when they wish to suggest a close tie between the sentences, are not subject to a publisher's house style, have a relaxed approach to punctuation, or are not certain about the use of commas and periods. The practice is often adopted to represent disjointed speech or thought, as in: ‘We didn't stay long, it gave me the creeps’ and ‘I made myself some food, gosh, I was hungry’ ( Somerset Maugham, The Razor's Edge, 1967). See PARATAXIS.(3) Sometimes, to separate a subordinate clause from a main clause, especially when the subordinate clause comes first or when the sentence is long: I decided to wait because I thought you would soon arrive (comma optional before because); Because I wanted to see them, I decided to wait (comma usual); However much you try, you will find it difficult (comma usual). Use of a comma is especially common after a conditional clause which is placed first (If you want to come, you must hurry) but less common in short sentences in which it is placed second (You must hurry if you want to come: a comma before if in this example would give extra emphasis to what follows, in the manner of other examples given above). A comma is normally required when the subordinate clause is participial: Feeling unwell, they decided to stay at home.(4) To separate a RELATIVE CLAUSE from its antecedent when the clause is giving incidental information rather than identifying the antecedent: compare the car, which was standing in the road, was stolen and the car which was standing in the road was stolen. In the first example, the position of the car is extra information that is not essential to the main sense; in the second, it identifies the car being referred to.(5) A comma is often used to introduce direct speech in place of a colon (I then said, ‘Why don't you come in?’), and is usual in resuming direct speech after a break, if the direct speech continues a sentence (‘Go away,’ I said, ‘and don't ever come back’). See QUOTATION MARKS.
The clarifying commaThe comma is also used to prevent ambiguity or misunderstanding. If the comma is removed from the following examples difficulties may arise, at least for a moment: With the police pursuing, the people shouted loudly; He did not want to leave, from a feeling of loyalty; However, much as I would like to, I cannot come.
Other usesThe comma is also used in various ways not directly associated with punctuation, such as: (1) To indicate thousands in numerals, beginning from the right: 5,324,768. (2) Traditionally, to separate a street number from the name of the road or street in addresses (a role that has diminished greatly in recent years): 24, High Street, probably more commonly 24 High Street. It has also traditionally been used to end each stage of an address, a practice that also appears to be obsolescent. (3) In letters, at the end of the initial greeting (Dear James, …; Dear Mrs Taylor, …), and at the end of the concluding formula (Yours sincerely,).
com·ma / ˈkämə/ • n. 1. a punctuation mark (,) indicating a pause between parts of a sentence. It is also used to separate items in a list and to mark the place of thousands in a large numeral. 2. a butterfly (genus Polygonia, family Nymphalidae) that has wings with irregular, ragged edges and typically a comma-shaped mark on the underside of each hind wing. Its numerous species include the common eastern comma (P. comma) of eastern North America.