Sentential relative clause

A sentential (also called connective) relative clause does not refer to a preceding noun; it rather comments on the whole preceding clause or sentence:

The streets were empty, which was unusual for this time of day. (The fact that the streets were empty was unusual.)

In conversation, a sentential relative clause may also be interjected by another speaker:

"Then he goes on to say how much he appreciates all the effort we've put into it." "Which is strange because he never seemed to care much about what we do."