My mother's parents were Nan and Grandad; my father's were Granny and Grandad. Grandads were distinguished by appending an appropriate surname in case of ambiguity. All four now gone.
My mother is called Nanny by those suitably qualified.
My father had gone AWOL by the time anyone might have offered him a corresponding name, and the only one of my brothers who has any contact with him has no contact with his daughter.
So... what about everybody else? What does your family call them?