That vs. Which vs. Who

That, which, and who are words (relative pronouns) which signal to a person who is reading or writing that a group of words form a dependent or subordinate clause. When should each be used?

First of all, use who only with people and sometimes with animals that seem almost human.

• I noticed a man who had tattoos of shankes adorning his forehead and cheeks. 
• The dog who greeted them with a toothy grin was hers.  

But what about that and which? Writers often struggle in their choices between the two. There are a few easy rules to help lessen the confusion which these two simple words create.

Use that to refer to animals and things and anonymous groups of people.

• The pet shop charged twenty dollars for the guinea pig that I wanted.
• The box that she chose was decorated with sequins and ribbons.
• We enjoyed the band that played at intermission.

Use that with restrictive clauses, so called because they restrict the meaning to a specific thing. The previous three examples are restrictive clauses because they point to a particular guinea pig, box, and band. Note that commas are not used with restrictive clauses.

Use which to refer to animals and things. Which can be used for both restrictive and nonrestrictive clauses.

• The lecture which he attended was interesting and informative.
• The bird, which is on the endangered species list, is an eagle.
• Thank you for the gift of chocolates, which I truly enjoyed.

In the above examples, which is first used with a nonrestrictive clause, as shown both by the reference to a particular lecture and the lack of a comma usage (the word that would also be acceptable in the sentence.).

In the next two sentences, commas are used because the clauses that they introduce could be omitted, and the meanings of the sentences would not be changed.

Some authorities, including the APA Style Manual, recommend that, to avoid confusion, which be used only for nonrestrictive clauses.