What kind of verb form is used to indicate the perfect progressive tense?

The perfect progressive tense uses the past participle of the main verb combined with the auxiliary verb "have" in its past participle form ("had") and the present participle (the "-ing" form) of the main verb.

Here's the structure:

Had + been + verb-ing

For example:

* I had been working for three hours before I took a break.

* They had been playing in the park all afternoon.

The perfect progressive tense emphasizes that an action started in the past, continued for a period of time, and was completed before another action or point in time.