I only met my maternal grandmother once - I referred to her as "Grandma". Never knew my maternal grandfather. My father's mother was also "Grandma", although it was usually elided to "Gramma". My father's father was "Granda" - elided from "Grand-da", "da" being common in Ireland where we would say "dad". My father gets "Grampa", elided from "Grandpa" from his grandchildren.