Here's why:
* "Which" is a relative pronoun used to introduce a restrictive clause. Restrictive clauses are essential to the meaning of the sentence and cannot be removed without changing the meaning.
* Restrictive clauses are not set off by commas.
Example:
* Incorrect: The book, which I borrowed from the library, is due tomorrow.
* Correct: The book which I borrowed from the library is due tomorrow.
Explanation: The phrase "which I borrowed from the library" is essential to identify which book is being discussed. Removing it would leave the sentence ambiguous.
However, there are exceptions:
* "Which" can introduce a non-restrictive clause, in which case a comma is used. Non-restrictive clauses add extra information but are not essential to the meaning of the sentence.
* "Which" can sometimes be used to introduce a restrictive clause after a proper noun, in which case a comma may be used depending on the context.
It's best to check the context and determine whether the clause is restrictive or non-restrictive to decide whether a comma is needed.