# which or that?

That usually introduces essential information in what is called a "restrictive clause." Which introduces extra information in a "nonrestrictive clause."

1. Example from a recent email:
• "I am offering a new class, Email Intelligence, that/which may be an excellent fit for your training needs and budget."
• Does the clause (in red) introduce information that is essential to knowing which Email Intelligence class?
• No. The clause provides extra information, so which is correct.
2. Revised example:
• "Among my new programs, I am offering a class that/which may be an excellent fit for your training needs and budget."
• Does the clause (in red) introduce information that is essential to knowing which class?
• Yes. The clause tells which class--a class that may be an excellent fit. Therefore, that is correct.