Examples

Ten out of ten dentists agree that Mark needs to go buy all new toothbrushes. In this example, which is a relative adjective because it introduces the subordinate clause which toothbrush he used to clean the grout. Since he's choosing between a set number of toothbrushes, which is the way to go.

In this example, what is a relative adjective that begins the subordinate clause what musical the drama club stages this spring. Given that Mr. J has forced the drama club to perform Sweeney Todd for seven straight years, it's a safe bet that he'll have them do it again this year—but since there's no set number of options, we go with "what" instead of "which."

Here, whichever is a relative adjective that kicks off the dependent clause whichever ugly Christmas sweater you decide to wear. It's not the holiday season until you've been invited to 438 Ugly Christmas Sweater parties, and whichever basically means "any and all."