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4 Punctuation

4.3 Comma

4.3.1 Restrictive and non-restrictive uses

There are two kinds of relative clause, which are distinguished by their use of the comma. A defining or restrictive relative clause cannot be omitted without affecting the sentence’s meaning. It is not enclosed with commas:

  • Identical twins who share tight emotional ties may live longer

  • The people who live there are really frightened

A clause that adds information of the form and he/she is, and it was, or otherwise known as (a non-restrictive or non-defining clause) needs to be enclosed with commas. If such a clause is removed the sentence retains its meaning:

  • Identical twins, who are always of the same sex, arise in a different way

  • The valley’s people, who are Buddhist, speak Ladakhi

Note that in restrictive relative clauses either which or that may be used in British English, but in non-restrictive clauses only which may be used. Restrictive:

  • They did their work with a quietness and dignity which he found impressive

  • They did their work with a quietness and dignity that he found impressive


  • This book, which is set in the last century, is very popular with teenagers

In US English which is not used for restrictive clauses, unlike British usage of which or that.

Similar principles apply to phrases in parenthesis or apposition. A comma is not required where the item in apposition is restrictive or defining—in other words, when it defines which of more than one people or things is meant:

  • The ancient poet Homer is credited with two great epics

  • My friend Julie is absolutely gorgeous

Note, however, that when the name and noun are transposed commas are then required:

  • Homer, the ancient poet, is credited with two great epics

  • Julie, my friend, is absolutely gorgeous

Use a comma or commas to mark off a non-defining or non-restrictive word, phrase, or clause which comments on the main clause or supplies additional information about it. Use a pair of commas when the apposition falls in the middle of a sentence; they function like a pair of parentheses or dashes, though they imply a closer relationship with the surrounding text:

  • I met my wife, Dorothy, at a dance

  • Their only son, David, was killed on the Somme

Ensure that a parenthetical phrase is enclosed in a pair of commas; do not use one unmatched comma:

  • Poppy, the baker’s wife, makes wonderful spinach and feta pies

  • Poppy, the baker’s wife makes wonderful spinach and feta pies

Do not use a comma when what follows has become part of a name:

  • Dave, the builder from down the road, said …

  • Bob the Builder

4.3.2 Comma splice

A comma alone should not be used to join two main clauses, or those linked by adverbs or adverbial phrases such as nevertheless, therefore, and as a result. This error is called a comma splice. Examples of this incorrect use are:

  • I like swimming very much, I go to the pool every week

  • He was still tired, nevertheless he went to work as usual

This error can be corrected by adding a coordinating conjunction (such as and, but, or so) or by replacing the comma with a semicolon or colon:

  • I like swimming very much, and go to the pool every week

  • He was still tired; nevertheless he went to work as usual

or by splitting it into two full sentences.

4.3.3 After an introductory clause or adverb

When a sentence is introduced by an adverb, adverbial phrase, or subordinate clause, this is often separated from the main clause with a comma:

  • Despite being married with five children, he revelled in his reputation as a rake

  • Surprisingly, Richard liked the idea

This is not essential, however, if the introductory clause or phrase is a short one specifying time or location:

  • In 2000 the hospital took part in a trial involving alternative therapy for babies

  • Before his retirement he had been a mathematician and inventor

Indeed, the comma is best avoided here so as to prevent the text from appearing cluttered.

Use a comma when a preposition is used as an adverb:

In the valley below, the villages looked very small

If commas are omitted, be vigilant for ambiguities:

In 2000 deaths involving MRSA in males increased by 66 per cent

Prefer In 2000, … or recast the sentence.

A comma should never be used after the subject of a sentence except to introduce a parenthetical clause:

  • The coastal city of Bordeaux is a city of stone

  • The primary reason that utilities are expanding their non-regulated activities is the potential of higher returns

  • The coastal city of Bordeaux, is a city of stone

  • The primary reason that utilities are expanding their non-regulated activities, is the potential of higher returns

When an adverb such as however, moreover, therefore, or already begins a sentence it is usually followed by a comma:

  • However, they may not need a bus much longer

  • Moreover, agriculture led to excessive reliance on starchy monocultures such as maize

When used in the middle of a sentence, however and moreover are enclosed between commas:

  • There was, however, one important difference

However is, of course, not followed by a comma when it modifies an adjective or other adverb:
  • However fast Achilles runs he will never reach the tortoise

4.3.4 Commas separating adjectives

Nouns can be modified by multiple adjectives. Whether or not such adjectives before a noun need to be separated with a comma depends on what type of adjective they are. Gradable or qualitative adjectives, for example happy, stupid, and large, can be used in the comparative and superlative and be modified by a word such as very, whereas classifying adjectives such as annual, mortal, and American cannot.

No comma is needed to separate adjectives of different types. In the following examples large and small are qualitative adjectives and black and edible are classifying adjectives:

  • a large black gibbon native to Sumatra

  • a small edible fish

A comma is needed to separate two or more qualitative adjectives:

  • a long, thin piece of wood

  • a soft, wet mixture

No comma is needed to separate two or more classifying adjectives where the adjectives relate to different classifying systems:

  • French medieval lyric poets

  • annual economic growth

  • randomized controlled trial

Writers may depart from these general principles in order to give a particular effect, for example to give pace to a narrative or to follow a style, especially in technical contexts, that uses few commas.

4.3.5 Serial comma

The presence or lack of a comma before and or or in a list of three or more items is the subject of much debate. Such a comma is known as a serial comma. For a century it has been part of Oxford University Press style to retain or impose this last comma consistently, to the extent that the convention has also come to be called the Oxford comma. However, the style is also used by many other publishers, both in the UK and elsewhere. Examples of the serial comma are:

  • mad, bad, and dangerous to know

  • a thief, a liar, and a murderer

  • a government of, by, and for the people

The general rule is that one style or the other should be used consistently. However, the last comma can serve to resolve ambiguity, particularly when any of the items are compound terms joined by a conjunction, and it is sometimes helpful to the reader to use an isolated serial comma for clarification even when the convention has not been adopted in the rest of the text. In

  • cider, real ales, meat and vegetable pies, and sandwiches

the absence of a comma after pies would imply something unintended about the sandwiches. In the next example, it is obvious from the grouping afforded by the commas that the Bishop of Bath and Wells is one person, and the bishops of Bristol, Salisbury, and Winchester are three people:
  • the bishops of Bath and Wells, Bristol, Salisbury, and Winchester

If the order is reversed to become

  • the bishops of Winchester, Salisbury, Bristol, and Bath and Wells

then the absence of the comma after Bristol would generate ambiguity: is the link between Bristol and Bath rather than Bath and Wells?

In a list of three or more items, use a comma before a final extension phrase such as etc., and so forth, and the like:

  • potatoes, swede, carrots, turnips, etc.

  • candles, incense, vestments, and the like

It is important to note that only elements that share a relationship with the introductory material should be linked in this way. In:

  • the text should be lively, readable, and have touches of humour

only the first two elements fit syntactically with the text should be; the sentence should rather be written:
  • the text should be lively and readable, and have touches of humour

4.3.6 Figures

In general text most styles use commas to separate large numbers into units of three, starting from the right:

  • £2,200

  • 2,016,523,354

but it will depend on the nature of the material; for example technical texts use a thin or non-breaking space as a thousands separator; for more about numbers see Chapter 11 and 14.1.3.

4.3.7 Use in letters

In formal contexts commas are used after some salutations in letters and before the signature:

  • Dear Sir, …

  • Yours sincerely, …

US business letters use a colon after the greeting. On both sides of the Atlantic, however, punctuation is now sometimes omitted, particularly after email salutations (if any) and in informal writing. For more on addresses see 6.2.4.

4.3.8 Other uses

A comma is often used to introduce direct speech: see 9.2.2.

A comma is placed after namely and for example when part of a parenthetical phrase:

  • We categorized them into three groups—namely, urban, rural, or mixed

  • Take, for example, the shutting of a small girl in a locker by three older girls

but no comma is needed when introducing an item or list:

  • The theoretical owners of the firm, namely the shareholders …

  • If they are off the road, for example on a grass verge, the district council then has to be notified

A comma is generally required after that is. Oxford style does not use a comma after i.e. and e.g. to avoid double punctuation, but US usage does (see 21.2.1).

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