reinforcement plate
An extra paper plate that you grab to put under the first to add reinforcement (in case it gets soggy, deformed, etc). Some of the time, one will keep the reinforcement plate, and only replace the top plate. The same can apply to cups.
Bob: Hey, Steve! I can't help but notice that you keep grabbing 2 or more plates everytime you come back up to get some mashed potatoes.
Steve: Oh, those are reinforcement plates.
Bob: Okay, but could you stop eating all the chicken?
Steve: Oh, those are reinforcement plates.
Bob: Okay, but could you stop eating all the chicken?