"The front door opened and Biff walked out. / 'Biff!' the elderly woman's voice called from inside the Tannen house. 'Where are you going, Biff?' / Biff started to walk quickly away from the house. / 'To get my car, Grandma!' he called over his shoulder. / 'But when are you coming back, precious?' his grandma whined. 'My feet hurt, and I want you to rub my toes some more. And put polish on them!' / Biff waved violently back at the house. / Marty called back to Doc on the walkie-talkie. 'Never mind, Doc. Biff lives here all right.' "
Gertrude was known to be an extremely unpleasant woman, and did not seem to encourage visitors. This was supported by the fact that a large handwritten sign at the gate read NO TRESPASSING / VIOLATORS WILL BE PROSECUTED / THIS MEANS YOU!!, while a handwritten sign reading KEEP OFF THE GRASS! was also stuck into the front lawn.
Behind the scenes
Gertrude Tannen was never shown on-screen, but her voice was provided by Thomas F. Wilson, notably where she is yelling at her grandson Biff (also played by Wilson).
Tom Wilson stated in an interview that the incessant nagging from Biff's grandmother may be the primary reason Biff grew up to be so mean. She could be heard yelling to Biff the night of the Enchantment Under the Sea dance.