Relative clause

Relative clauses are essentially those clauses (groups of words with a verb) that start with a word like who or which. (In traditional terms these are relative pronouns; in more modern grammars, they may be called wh-words.) Relative clauses are also called adjectival clauses.

Problems in punctuation arise with relative clauses because they are used in two different ways.

• They can be used to define what you are talking about. For example, "This is the friend who I told you about yesterday" [or "...about whom I told you," in academic English.] If I just said "This is the friend" you would not know whom I meant; you'd ask yourself 'Which friend?' So the word 'friend' is defined by the relative clause which follows it. This is a defining relative clause, also - and increasingly - called a restrictive relative clause.

• Alternatively, relative clauses can be used to add extra information. For example, "This is Mr Smith, who [or whom, in academic English] I told you about yesterday." [Even better in academic English "about whom I told you yesterday".] Here you know who I mean - Mr Smith; you have his name. The fact that I said something about him yesterday is an extra fact. It does not define him. This is a non-defining relative clause. Another example is when we add a titbit of information about someone we presume our audience knows all about. "Churchill, whose first career was in the army, was in charge of the Admiralty in 1914." Here, I am presuming that all my readers know about the Prime Minister of the U.K. during the Second World War: "whose first career was in the army" does not define him. I only said it in a hope of interesting my reader, more for stylistic reasons than for genuinely historical ones. Again, it is a non-defining relative clause, or non-restrictive relative clause..

The difference in practical terms for writers is this:

• With defining relative clauses, don't use commas.

• With non-defining relative clauses, use a pair of commas. These are essentially like mini-brackets. (Both brackets and pairs of commas act to separate extra bits of information from the main sentence. This kind of usage can be called parenthetical.)

For more on some particular areas, see That (or) which.