It’s more a question of what people in your family are most likely to understand. For me, it changes according to context. If I’m talking about something that pertains to me or my relationship with my parents, I say Mom or Dad. If I’m talking about something pertaining to them or their relationship with my parents, I say Grandma/Grandpa. Actually, it’s Grandma Sarah or Nanners or Grammie — they all called her something different. :)
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