In the first sentence, the fact that the chairs don’t have cushions is something
essential to the sentence; it is a clause that directly identifies the subject. (Which
chairs? The ones that don’t have cushions.) In the second sentence, the fact that
the chairs are found in many places of work is treated as extra information.
(Which chairs? The chairs that are uncomfortable to sit on, which happen to be in
many places of work.)
In both of these cases, that identifies a restrictive clause with
essential information, while which identifies a nonrestrictive clause with
nonessential information.
Interchangeable Restrictive and Nonrestrictive Elements:
For further thought: What happens when restrictive and
nonrestrictive clauses are interchangable? The meaning of the
sentence can change depending on whether a restrictive or
nonrestrictive element is used.
For example:
He helped the native guides who were sick with malaria.
No comma is used before “who,” making this a restrictive clause. This tells the
reader that not all the guides had malaria.
He helped the native guides, who were sick with malaria.
Putting the comma before “who” makes what follows a non-restrictive clause. In
addition, this changes the sentence to mean that all the guides had malaria.
Another example:
My brother, Ken, worked for a TV ratings company.
Adding commas around “extra” information in this sentence indicates that I have
only one brother, and his name is Ken. Due to the fact that I only have one
brother, his name is a nonrestrictive element or nonessential to the meaning of