The past perfect expresses an action that took place in the past before another action was completed. For example, they had left or she had called.
The trapassato prossimo is formed with the imperfect of avere or essere, plus the past participle of the verb. The agreement of the past participle must follow the same rules as those for the passato prossimo, meaning that verbs conjugated with the auxiliary verb essere must show agreement in gender and number in the past participle. Take a look at the table below, which shows example verbs using both avere and essere.