The relative pronouns which and that are easily confused, and there is no definitive correct usage; furthermore, these words are often used differently in speech. However, the two words are generally not used in an exactly identical fashion in writing. Which often refers to the idea presented by the entire previous clause, and the word is commonly used to introduce non-restrictive clauses (see below).

A claim was made that the pendant belongs to a “female hoard”, which is one of the claims examined in this thesis.

The phrase “which is one of the claims examined in this thesis” is called a non-restrictive or non-defining clause, because this clause does not add crucial information to the main clause. This type of clause is (usually) set off by a comma.