Rules for comma usage

By Marina Pantcheva

The comma is perhaps the most puzzling mark of punctuation. The rules for using commas are so numerous and can seem so arbitrary that one often wishes one could dispense with them once and for all. Really, are the commas so vital in the sentence below?

(1) Historically the comma is derived from the diagonal slash which was used to indicate a pause. [incorrect]

(2) Historically, the comma is derived from the diagonal slash, which was used to indicate a pause. [correct]

It seems, indeed, that the commas can be removed in example (1). However, their presence becomes justified if we read the sentence aloud — we make a short pause after the words historically and slash, precisely the places where the commas should be. A useful rule of thumb is to place commas where one makes a pause in speech.

Rule of thumb: a comma indicates a pause in speech.

When in doubts then, read the sentence aloud. If you pause at some place, insert a comma to mark the pause.

Still, commas are more than simple pause-markers; they help the reader understand the structure of the sentence and resolve ambiguity. Compare the two sentences:

(3) The students who passed the exam went on a fieldwork trip.

(4) The students, who passed the exam, went on a fieldwork trip.

The sentence without comma means that only those students who passed the exam went on a trip. The sentence with commas means that all students went on a fieldwork trip, and they all, by the way, passed the exam.

Below follow a few rules about how to use commas correctly.

Rule 1: Use commas in a series of three or more items.

Normally, the last item in the series is preceded by and, or, or nor.

(5) The new regulations concern students, research fellows, and post-doctoral researchers.

The comma placed before and (or, nor) is not obligatory, but it is recommended because it sometimes disambiguates the sentence.

(6) Tom’s favorite dishes are tomatoes, fish and chips, and toasts.  

Sentence (6) means that Tom likes fish and chips, and he also likes toasts. Without the comma before and it is unclear how to group the last three items.

(7) Tom’s favorite dishes are tomatoes, fish and chips and toasts.

dish 1: fish and chips, dish 2: toasts

or

dish 1: fish, dish 2: chips and toasts

Rule 2: Use a comma to separate two or more coordinate adjectives.

Coordinate adjectives describe the noun separately.

(8) It was a dark, stormy night.

(9) The new headmaster is a tall, good-looking man.

There are two easy ways for testing whether the adjectives are coordinate or not

  • and-test: if you can put and between the adjectives without changing the meaning, then the adjectives are coordinate, e.g., He is a tall and good-looking man.
  • reverse-test: if you can change the order of the adjectives without changing the meaning, then the adjectives are coordinate, e.g., He is a good-looking, tall man.

Non-coordinate adjectives should not be separated by a comma.

(10) Tom hates these complicated coordinate adjectives.

Separating complicated and coordinate by a comma is incorrect, as the two adjectives are not coordinate (coordinate complicated adjectives means something else).

Rule 3: Use a comma to separate independent clauses introduced by and, or, nor, but, yet, for, so (in the sense of “as a result”), as (in the sense of “because”), and while (in the sense of “at the same time”).

(11) The rain poured down, and John looked for a shelter.

(12) Marta handed in the application, but she was not satisfied.

Be careful to use a comma only when the part after and, or, nor, but, etc., is a full clause. If it is not, then no comma should be used.

(13) The rain poured down and ruined the parade.

(14) Marta handed in the application but did not like it very much.

An easy way to determine whether the second part of the sentence is a full clause is to see whether it has its own [subject]. If it does, then it is a full clause.  For instance, the second clause in example (11) has the subject John, different from the subject the rain in the first clause. Similarly, the second clause in example (12) has its own subject, she, which, accidentally, refers to the same person as the subject in the first clause (Marta). This plays no role; what matters here is that there is a second subject present.

For comparison, there is no second subject after and and but in examples (13)-(14).

NB! If the independent clauses are not joined by a conjunction, i.e., and, or, nor, but, etc., then the correct mark is the semicolon (;), see more under [semicolon].

Rule 4: Use a comma after an introductory phrase/word.

(15) In the event of disagreement about the duties, the work plan may be presented to the PhD Committee.

(16) Nevertheless, the students performed poorly.

(17) On Tuesday, the Prime Minister met the demonstrators.

Introductory phrases that need to be separated by a comma include:

1. Participial phrases (using the –ing form of the verb)

(18) Having finished the meeting, the committee members went back to their offices.

2. Infinitive phrases (beginning with to+verb)

(19) To access the wireless network on campus, you must know the password.

3. Prepositional phrases (beginning with a preposition, e.g., by, on, under, without, etc.)

(20)  Despite our differences, we collaborate well.

When the prepositional introductory phrase is short (less than 3-4 word), the comma can be omitted, but it is not wrong to use it.

4. Introductory words, such as however, consequently, hence, thus, nevertheless, etc.

(21) Consequently, the experiment failed.

Rule 5: Use a comma after a [dependent clause] preceding an [independent clause].

(22) If Smith accepts our conditions, we will agree to the proposal.

(23) Although Smith did not accept our conditions, we agreed to the proposal.

The dependent clause can provide the time, the condition, the reason, etc., for the event in the main clause. Commonly, dependent clauses begin with if, whether, because, although, since, when, while, unless, even though, whenever, etc., (follow this link for an exhaustive list).

Note that a dependent clause should be separated from an independent clause by a comma only when the dependent clause precedes the independent one. If the dependent clause follows the independent one, no comma is places before if, whether, because, although, since, when, while, unless, etc.

(24) We agreed to the proposal because Smith accepted our conditions. [correct]

(25) We agreed to the proposal, because Smith accepted our conditions. [incorrect]

But

(26) Because Smith accepted our conditions, we agreed to the proposal.  [correct]

Rule 6: Use commas before and after parts of the sentence that are not essential to its meaning.

Such non-essential parts can be words, phrases and entire clauses. An easy way to test whether a word, a phrase, or a clause is non-essential is to simply leave it out and see whether the message changes dramatically.

(27) Our experimental data, however, shows that the previous results are unreliable, at best.

(28) The audience, indifferent at the beginning, became more and more interested.

(29) In 1888, when my great grandmother was born, there was only one hospital in the entire country.

Do not use commas to separate essential parts of the sentence.

(30) People who live in glass houses should not throw stones. [correct]

(31) People, who live in glass houses, should not throw stones. [incorrect]

The part who live in glass houses is essential to the meaning since the only the people to whom this applies should not throw stones. The sentence in (31) implies that people generally live in glass houses.

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