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Adding commas around a relative clause can change the meaning of its sentence
THE ENGLISH RELATIVES
The English relative pronouns are who, whom, which, that and whose. They figure briefly in some other posts, e.g. 28. Pronoun Errors and 52. Participles Placed Just After their Noun. Here I wish to consider when relative pronouns should and should not be used with bracket-like commas (a comma type discussed under 50. Right and Wrong Comma Places). Most English grammar books explain this, but the explanations are rarely easy to follow, and I am hoping to do a little better.
The difference between having and not having commas with a relative clause is basically one of meaning. In the following sentences, for example, the word people does not stand for the same people each time:
(a) People who live in glass houses should not throw stones.
(b) People, who are the most intelligent of all creatures, sometimes act foolishly.
Sentence (a) is an English proverb meaning that people who deserve to be criticised should not criticise others. It is about some people only. Sentence (b), however, is about all people. The commas help to make this clear. Note, incidentally, that there are two commas involved. The only time when the second one is not necessary is when it coincides with the full stop at the end of the sentence, in which case the full stop is preferred.
TEXTBOOK EXPLANATIONS OF RELATIVE PRONOUN PUNCTUATION
Some rather unhelpful grammar book explanations say that two commas are needed when the information after the relative pronoun is “unnecessary” or “additional”. These words can be confusing because they are easily misunderstood: if information is unnecessary, why is it given at all? Other grammar books use words like “non-defining” for relatives with commas and “defining” for relatives without commas (I have also seen, instead of “defining”, the words “specifying”, “identifying”, “classifying” and “restrictive”). All of these names are useful only if you know what they mean to start with, and what is or is not being defined etc.
Beginning with the second of these questions, the person or thing that is or is not being defined etc by a relative clause is usually the person/thing expressed by the noun before the relative: people in the examples above. In answer to the first question, the meaning of words like “defining” may be better understood by separating the nouns before a relative into two types: those with plural meaning (whether plural in form or singular with non-singular meaning) and those with singular meaning.
RELATIVE PRONOUNS AFTER NOUNS WITH PLURAL MEANING
Plural meaning is understandable from most plural nouns and some singular ones. It is expressed by singular nouns when they represent a general quantity or group, as in Exercise preserves health or A pen is a weapon (for more about “general”, see 89. Using “the” with General Meaning). Such nouns usually lack an article if they are uncountable (like exercise above) or have a(n) if countable (like pen). The relative pronoun punctuation rule after nouns with plural meaning is that commas show all of the noun idea is meant, while no commas show some. When “some” is meant, the relative clause tells us which ones or which part. It is this idea of telling us which that words like “defining” try to communicate. Knowing the rule helps us to see that sentence (a) above is about some people and (b) is about all.
Sentences (a) and (b) are actually quite easy to understand because the meanings of “all” and “some” can be recognised even without considering the punctuation. Not all relative pronoun sentences are so easy though. Which of the following is correct?
(c) (The) reforms which Napoleon introduced were long lasting.
(d) The reforms, which Napoleon introduced, were long lasting.
In fact, either of these could be correct depending on what we mean. The first, with no commas, shows that some reforms (the subgroup introduced by Napoleon) were long lasting. The word the occurs at the start because the subgroup of reforms being given after it can be considered particular ones. In the second sentence, the two commas mean that no subgroup of reforms is being given, so we must understand that all of the reforms are meant.
But what is meant by the reforms? They are not “all reforms” (because that would be reforms), and they are not a subgroup resulting from which. They are, in fact, an already explained subgroup. The clue to this fact is another use of the, meaning “the ones that have just been explained”. In other words, we must look to the previous sentence to find out what the reforms are, and not to the relative clause. Thus, sentence (d) could be paraphrased as “All of the previously-defined subgroup of reforms (which were the same as Napoleon’s ones) were long lasting”.
A link between commas and the “all/some” distinction is found with some other English words besides relative pronouns. Interested readers are referred to the posts 52. Participles Placed Just After their Noun, 53. “As”, “Like” and “Such As”, 54. Words Before Lists 1 (Incidental) and 77. Apposition (Pairing of Same-Meaning Nouns).
RELATIVE PRONOUNS AFTER NOUNS WITH SINGULAR MEANING
Singular meaning is mostly expressed by non-general singular nouns, like this:
(e) Alexander the Great was struck by an illness that proved fatal.
In addition, singular meaning can be expressed by general singular nouns representing something unique. These may be ordinary nouns like the greenhouse effect (see 89. Using “the” with General Meaning) or “proper” nouns like London (see 47. Article Errors with Proper Nouns), like this:
(f) London, which is the capital of the UK, has a population of over seven million.
The punctuation rule after nouns with singular meaning is that commas show the noun idea to be the only one in existence, while no commas show it to be one of a group. When others exist, the relative pronoun without commas helps tell us which one of all the possibilities is meant. It is this idea of telling us which one that words like “defining” try to communicate.
In sentence (e), the absence of a comma before that suggests Alexander suffered many illnesses, and the words after that show how the one mentioned is different from the others. In (f), the comma before which suggests there is only one London, so that the following words are not showing which one is meant but are rather just describing that one.
The meaning created by the punctuation next to the relative pronoun in sentences (e) and (f) is again quite easy to see because it matches what we already know about the world. However, quite often our knowledge of the world will not help us. Consider this:
(g) The student cafe, which is on the campus, is always open.
Some colleges have just one student cafe, while others have more than one. In the case that the above sentence refers to, we have to rely on the commas to know that there is just one student cafe. Moreover, the same sentence could be written without commas, and then it would show that there was more than one student cafe.
In fact, even names like London can often be used without commas as well as with. This is because the same name is often possessed by different people or places. Consider this:
(h) The London that is in Canada is sometimes confused with the one in England.
In these cases, it is normal to have the before the name. If we removed the commas from sentence (f), so that the relative pronoun there was helping us to know which London was meant, again we would need to begin with the. It may be asked how we can ever suggest that there is only one London, as in (f), when in reality there are others. The answer is that speakers often use London (and other names) to mean “the one that we are familiar with” rather than “the only one”.
WHEN TO USE “which” AND WHEN TO USE “that”
Understanding the use of commas is vital for choosing correctly between which and that in relative clauses. The rule is simple enough:
Relatives Without Commas
Use which or that
Relatives With Commas
Beware of computer grammars that tell you to use only that without commas; the truth is that which is nearly always possible too (more on the unreliability of computer grammars is in the posts 68. & 69. How Computers Get Grammar Wrong) . It is only in some special sentences where which is not possible, such as those where the previous noun follows a superlative adjective:
(i) This is the best thing that could have happened.
Now here is a small exercise through which understanding of the above points may be checked.
EXERCISE: Decide which sentences below need two commas (or one comma and a full stop). Answers are given below.
1. Entebbe Airport which is near Kampala is Uganda’s main entry point.
2. The area of a circle can be calculated using pi which is the ratio of the circumference of a circle to its diameter.
3. The Prime Minister who is the head of the government is formally appointed by the President.
4. Students should be prepared to question a lecturer who is not clear.
5. Students should be prepared to question a lecturer whose job it is to answer questions.
6. A vowel is a sound which is made by the vocal chords while air is passing without obstruction through the mouth.
7. Traffic lights which control traffic by showing three different colours in a predictable sequence are similar to language.
8. Workers who firms cannot afford to pay have to lose their jobs.
Answers: 1 = commas after Airport and Kampala; 2 = no commas; 3 = no commas; 4 = no commas; 5 = comma after lecturer; 6 = no commas; 7 = commas after lights and sequence; 8 = no commas.