From Mechanics in Guide part of ABC of Plain Words by Sir E Gowers (1951)

The use of commas cannot be learned by rule. Not only does conventional practice vary from period to period, but good writers of the same period differ among themselves. Moreover stops have two kinds of duty. One is to show the construction of sentences — the "grammatical" duty. The other is to introduce nuances into the meaning — the "rhetorical" duty. "I went to his house and I found him there" is a colourless statement. "I went to his house, and I found him there" hints that it was not quite a matter of course that he should have been found there. "I went to his house. And I found him there". This indicates that to find him there was surprising. Similarly you can give a different nuance to what you write by encasing adverbs or adverbial phrases in commas. "He was, apparently, willing to support you" throws a shade of doubt on his bona fides that is not present in "He was apparently willing to support you".

The correct use of the comma — if there is such a thing as "correct" use — can only be acquired by commonsense, observation and taste. The best general advice is Fowler's:

Everyone should make up his mind not to depend on his stops.... It may almost be said that what reads wrongly if the stops are removed is radically bad: stops are not to alter the meaning but merely to show it up.

Difficult though it may be to describe the correct uses of the comma, there is general agreement among the authorities that certain uses are incorrect. A warning can usefully be given against the commonest of these. Here are some of them.

  1. The use of a comma between two independent sentences not linked by a conjunction. A heavier stop should always be used in this position, usually a semicolon:
    We wrote on the 12th May asking for an urgent report regarding the above contractor's complaint, this was followed up on the 24th May by a telephone call.
    You may not be aware that a Youth Employment Service is operating throughout the country, in some areas it is under the control of the Ministry of Labour and National Service and in others of the Education Authorities.

    There should be a semicolon after complaint in the first quotation and country in the second.

    The Department cannot guarantee that a licence will be issued; you should not therefore arrange for any shipment.
    I regret the delay in replying to your letter but Mr. X who was dealing with it is on leave, however, I have gone into the matter...

    There should be a full stop after issued in the first quotation and after leave in the second.

  2. The use of a comma to mark the end of the subject of a verb, or the beginning of the object. See ARRANGEMENT.

    It cannot be said to be always wrong to use a comma to mark the end of a composite subject, because good writers sometimes do it deliberately. For instance one might write:

    The question whether it is legitimate to use a comma to mark the end of the subject, is an arguable one.

    But the comma is unnecessary; the reader does not need its help. To use commas in this way is a dangerous habit; it encourages a writer to shirk the trouble of so arranging his sentences as to make their meaning plain without punctuation.

    I am however to draw your attention to the fact that goods subject to import licensing which are despatched to this country without the necessary licence having first been obtained, are on arrival liable to seizure. . .

    If the subject is felt to be so long that it needs a boundary post at the end, it would be better not to use the slovenly device of a comma but to rewrite the sentence in conditional form.

    . . if goods subject to import licensing are despatched . . . they are on arrival . .

    Postponement of the object may get a writer into the same sort of trouble.

    In the case of both whole-time and part-time officers, the general duties undertaken by them include the duty of treating without any additional remuneration and without any right to recover private fees, patients in their charge who are occupying Section 5 accommodation under the proviso to Section 5 (1) of the Act.

    This unlovely sentence obviously needs recasting. One way of doing this would be:

    The general duties undertaken by both whole-time and part-time officers include the treating of patients in their charge who are occupying Section 5 accommodation under the proviso to Section 5 (1) of the Act, and they are not entitled to any additional remuneration for it or to recover private fees.
  3. The use of one comma instead of either a pair or none. This very common blunder is more easily illustrated than explained:
    Against all this must be set considerations which, in our submission are overwhelming. (Omit the comma.)
    We should be glad if you would inform us for our record purposes, of any agency agreement finally reached. (Either omit the comma or insert one after us.)
    It will be noted that for the development areas, Treasury-financed projects are to be grouped together. (Either omit the comma or insert one after that.)
    The first is the acute shortage that so frequently exists, of suitable premises where people can come together. (Either omit the comma or insert one after shortage.)
    The principal purpose is to provide for the division between the minister and the governing body concerned, of premises and property held partly for hospital purposes and partly for other purposes. (Omit the comma.)
  4. The use of commas with "defining" relative clauses. Relative clauses fall into two main classes. Grammarians give them different labels, but defining and commenting are the most convenient and descriptive. If you say "The man who was here this morning told me that", the relative clause is a defining one; it completes the subject "the man", which conveys no definite meaning without it. But if you say: "Jones, who was here this morning, told me that", the relative clause is commenting; the subject "Jones" is already complete and the relative clause merely adds a bit of information about him which may or may not be important but is not essential to the definition of the subject. A commenting clause should be within commas; a defining one should not. This is not an arbitrary rule; it is a utilitarian one. If you do not observe it, you may fail to make your meaning clear, or you may even say something different from what you intend. For instance:
    I have to express regret for the error which occurred in printing.

    By not putting a comma after error the writer has made the relative clause a defining one. He implies that he regrets the error that occurred in printing, but has no regrets about any errors that occurred in other ways. That is not what he wanted to say. Both he and his correspondent know what the error is that they are writing about. It does not need defining. What the writer of the letter wants to do is to express his regret and to say that it is the printer's fault. To do this he must put a comma after error, and make the relative clause a commenting one.

    Here is an example of the opposite mistake:

    I have made enquiries, and find that the clerk, who dealt with your enquiry, recorded the name of the firm incorrectly.

    The relative clause here is a defining one. The comma turns it into a commenting one and implies that the writer has only one clerk. The truth is that one of several is being singled out; and this is made clear if the commas after clerk and enquiry are omitted.

    The same mistake is made in:

    The Ministry issues permits to employing authorities to enable foreigners to land in this country for the purpose of taking up employment, for which British subjects are not available.

    The grammatical implication of this is that employment in general is not a thing for which British subjects are available. An instruction book called "Pre-aircrew English" supplied during the war to airmen in training in one of the Dominions, contained an encouragement to its readers to "smarten up their English". This ended:

    Pilots, whose minds are dull, do not usually live long.

    The commas convert a truism into an insult.

  5. The insertion of a meaningless comma into an "absolute phrase".

    An absolute phrase (e.g. "then, the work being finished, we went home") always has commas round it. But there is no sense in the comma that so often carelessly appears inside it.

    The House of Commons, having passed the third reading by a large majority after an animated debate, the bill was sent to the Lords.

    The insertion of the first comma leaves the House of Commons in the air waiting for a verb that never comes. See ING ENDINGS.

    A comma was at one time always used to introduce that clauses.

    It is a just though trite observation, that victorious Rome was itself subdued by the arts of Greece.(Gibbon.)
    The true meaning is so uncertain and remote, that it is never sought. (Johnson.)
    The author well knew, that two gentlemen . . . had differed, with him. (Burke.)

    We are more sparing of commas nowadays, and this practice has gone out of fashion, though some good writers still put commas in that position occasionally.

As to the legitimate uses of commas, it is unprofitable, for the reasons explained, to try to enumerate them: they can best be learned by observation. But there are a few points on which some guidance may be useful:

  1. Words and phrases in series.

    In such a sentence as:

    The company included Ambassadors, Ministers, Bishops and Judges

    commas are always put after each item in the series up to the last but one, but practice varies about putting a comma between the last but one and the and introducing the last. Neither practice is wrong. Those who favour a comma (a minority, but gaining ground) argue that, since a comma may sometimes be necessary to prevent ambiguity, there had better be one there always. Supposing the sentence were:

    The company included the Bishops of Winchester, Salisbury, Bristol, and Bath and Wells

    the reader unversed in the English ecclesiastical hierarchy needs the comma after Bristol in order to sort out the last two bishops. Without it they might be, grammatically and geographically, either (a) Bristol and Bath and (b) Wells, or (a) Bristol and (b) Bath and Wells. Ambiguity cannot be justified by saying that those who are interested will know what is meant and those who are not will not care.

    Where the series is of adjectives preceding a noun, it is a matter of taste whether there are commas between them or not:

    A silly verbose pompous letter, and A silly, verbose, pompous letter.

    are equally correct. The commas merely give a little emphasis to the adjectives. Where the final adjective is one that describes the species of the noun, it must of course be regarded as part of the noun, and not be preceded by a comma. Thus:

    A silly, verbose, pompous official letter.
  2. Commas for emphasis or clarity.

    Commas in pairs give a parenthetic effect to what they enclose which may vary from a true parenthesis ("These things, our correspondent says, have caused widespread discontent") or an explanation in apposition ("Mr. Jones, the Secretary, said . . . ") to the emphasising of an adverb or adverbial phrase:

    The amendment was, fortunately, ruled out of order.
    We may, some day, get a decision.

    The notes of relief in the first and of hopelessness in the second are emphasised by the commas.

    Or the effect of commas may be to emphasise the subject of the sentence "He, however, thought differently ". The commas underline he.

    Except for emphasis, or sometimes for clarity, it is better not to comma-off adverbs and adverbial phrases. Many writers and more compositors and typists habitually so treat such words as however, moreover, therefore, indeed, of course, no doubt, perhaps, and even also and too. Not only is this unnecessary but it throws away a convenient means of adding emphasis when emphasis is needed.

    Commas are sometimes needed to prevent ambiguity by showing what an adverb qualifies. However is a word that is specially likely to lead a reader astray. For instance, Burke writes:

    The author is compelled, however reluctantly, to receive the sentence pronounced on him in the House of Commons as that of the Party.

    The meaning of this sentence would be different if the comma after reluctantly were omitted, and one inserted after however.

    The author is compelled, however, reluctantly to receive, etc.

    Other adverbs may need similar clarification. This example is taken from Lord Dunsany:

    I am going to Dublin perhaps, with Murphy,

    which does not mean the same as

    I am going to Dublin, perhaps with Murphy.

    Adverbs and adverbial phrases standing at the beginning of sentences are often given superfluous commas:

    In their absence, it will be desirable . . .
    Nevertheless, there is need for special care . . .
    In practice, it has been found advisable . . .

    Those commas cannot be said to be wrong; the use of commas in this way is frequent in good writing. But they are unnecessary, and therefore we are better without them unless they are needed to prevent the reader from going off on a wrong scent, as in:

    A few days after, the Minister of Labour promised that a dossier of the strike would be published.
    Two miles on, the road is worse.
  3. Other uses to prevent ambiguity.

    Ambiguities of various kinds, besides those already mentioned, can often be removed by punctuation. Sometimes this is legitimate, sometimes not. The writer must not forget Fowler's dictum: "It may almost be said that what reads wrongly if the stops are removed is radically bad". He should consider whether the comma is felt to be naturally placed or to be logically intrusive. Apart from the uses already referred to in marking where the subject ends and the predicate begins (generally illegitimate) and in showing what an adverb qualifies (generally legitimate) the most common use of the comma as a clarifier is to show that what follows it refers not to what immediately precedes it but to something further back. William Cobbett, in the grammar that he wrote for his young son, pointed out that "You will be rich if you be industrious, in a few years" did not mean the same as "you will be rich, if you be industrious in a few years". The comma that precedes the adverbial phrase in a few years indicates that that phrase refers not to "if you be industrious" but to the whole clause "you will be rich if you be industrious". As usual, the device is clumsy. The proper way of writing the sentence is "You will be rich in a few years if you be industrious". Arrange your words in the right order and you will not need these artificial aids.