When I was born, my living grandparents were my paternal grandmother and my maternal grandfather. My grandmother was "Grandmommy." Everyone called my grandfather "Grandaddy" except for me. I called him "Grandpa." Well, when I was older. When I was little, I found out what I thought was his first name was actually his middle name. No one ever used his first name so, when I read it on his suitcase tag, it was like I found out a huge secret. To a 6yo, that's super exciting so I called him by it and he let me.
I want my grandchildren to call me Madea.
ETA: Oh and I just remembered. My grandmother's last husband was called "Uncle Firstname." I have NO idea why we had to call him uncle...maybe because he wasn't any of our grandfathers? IDK.