Which
When a clause is descriptive or parenthetical and can be removed from the sentence without changing the reader’s understanding of the noun, use which and enclose the clause with commas.
The World Bank, which has 184 members, has its headquarters in Washington, DC. |
There is only one World Bank, so the information in the which clause is merely descriptive and does not affect the reader’s understanding of the noun, “the World Bank.”
The July 14 policy paper, which Susan introduced at yesterday’s meeting, is under review. |
Because the reader already understands that the policy paper under review is the one dated July 14, the clause “which Susan introduced at yesterday’s meeting” is simply additional information; therefore, which should be used because the clause it introduces is not essential to the reader’s understanding of the noun.