English Corner 7: that and which clauses
The simple rule defining which personal pronoun should be used (people who, animals which, things that) only applies to what are usually known as that clauses. Better names for them (since not only that, but which and who can also introduce them) are defining, limiting and restrictive clauses, since they define, limit or restrict in some way the object described. Grammatically they are relative subordinate adjectival clauses and they are not separated from the object by a comma. In contrast to that clauses, are which clauses, better called non-defining, non-limiting, non-restrictive or descriptive clauses, since they describe in some way, but do not limit or define, the object. They function grammatically as relative coordinate main clauses and are separated from the object by commas.
Defining clauses introduced by that and who
• In [The microscope that has a broken lens needs to be repaired] the word that limits the meaning of microscope to (defines it as) the one with a broken lens. The sentence implies one thing: only the microscope with the broken lens needs to be repaired.
• In [Students dislike professors who show favouritism], the word who limits the meaning of professors to (defines them as) those who show favouritism. It too, implies one thing: students dislike only the professors who show favouritism.
Defining clauses not introduced by that or who
• In rare cases, that clauses are not even introduced by that or who, but lack of a comma indicates that a defining clause is being used. In [Androgens had no effect as the doctor claimed], the word as limits the meaning of doctor to (defines him as) the one claiming that androgens do have an effect. This implies only one thing: the results do not confirm what the doctor claimed.
Non-defining clauses introduced by which and who
• In [The microscope, which has a broken lens, needs to be repaired], the phrase within the commas can be omitted from the sentence without a change in meaning, as the essence of the statement is [The microscope needs to be repaired], with the phrase within commas merely adding a description of the object. This sentence provides two statements [The microscope needs to be repaired] and [The microscope has a broken lens].
• Similarly, in [Students dislike professors, who show favouritism], the phrase after the comma can be omitted without a change in meaning, the essence being [Students dislike professors], followed by a description of professors as showing favouritism. It too, provides two statements [(all) Students dislike professors) and [(all) Professors show favouritism].
Non-defining clauses not introduced by which or who
• Some which clauses need not be introduced by which or who, but the presence of a comma indicates its descriptive nature. In [Androgens had no effect, as the doctor claimed], the phrase after the comma can be omitted without a change in meaning, the essence being [Androgens had no effect]. The sentence provides two statements, the result [Androgens had no effect] and an inference [The doctor claimed that androgens would have no effect].
By Dr Trevor G Cooper (email@example.com)