The past perfect progressive shows that an action started in the past and continued up to another time or action, also in the past. It does not specify if the action continues after that point. This is a very uncommon verb form: less than .1% of verbs in spoken English are in this form.
Continue reading to learn how to form the past perfect continuous and when to use it, including loads of examples and charts!
Just to be clear, continuous and progressive are just different names for this same thing. In some countries, one word is more common, but continuous is more common around the world, so we use that!