Grandmother & Grandfather?
Grosspapa
Gran & Gramps
Mother Johnson & Papa
Grampa Smith & Grampa Jones
Gumba & Meemaw
Big Pops & Ma Big Pops
…
Unfortunately, I call my grandparents “dead”. That’s part of the territory when you’re an old fart, I guess. 
My kids call their’s Grandma and Grandpa. Which is what I used to call mine.
Grandma and Grandpa. No last names, as the grandparents were usually not there at the same time, so there was no reason for anyone to wonder who I meant by either name. For a while I did amplify my father’s stepmother to “Grandma Gerrie” because it seemed to hurt my maternal grandmother’s feelings for me to just call her Grandma. I still often refer to her as my Grandma Gerrie when talking about her, to avoid confusion with Mom’s mother. (This isn’t a problem with my grandfathers, since Dad’s father died when I was seven, and I’ve never called Grandma’s new husband by anything but his name. If I’m talking about something Grandpa did or said, people already know I’m talking about Mom’s father.)
Grandma Gerrie’s other grandkids use Grandmother and Grandfather. My husband’s family use Mamaw and Papaw for his grandparents, and Granny and Pappy for his great-grandparents.
I call mine Nana and Grandda.
I called my mum’s Grandma “Gran”
I call both my mum’s mum and dad’s mum “Grandma.” However if I’m talking ABOUT one of them I’ll somtimes add their first name on the end to avoid confussion.
My dads dad died when I was about 4, so I don’t remember what I called him.
My mum’s mum’s first husband I called “Grandad first name” (my biological grandad) However he died 2/3 years ago.
I called my grandma’s second husband “Grandad” (my non biological grandad. They got married before I was even born though.) Sadly he died last month.
Confused? Me too!
I call my one remaining grandparent Gandy or The Gand. It’s because the first grandchild couldn’t say “granddad” and used “gandy” instead. I like it very much and intend to get my (future) children to call my dad “gandy”.
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Vicious Old Bastard
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Lesbian Control-Freak
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Air-Headed Commie
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Whining Psycho
Ma & Papa
MawMaw & Pawpaw
pretty everyday common I guess.
By the time I was born both of my dad’s parents had already passed on, so the only grandparents I’ve ever known are the ones on my mother’s side, so there’s never been a reason for me to make a distinction between two sets of grandparents. For as long as I can remember I’ve always called them Grandma and Grandpa. They’re both still alive, but my grandma has Alzheimer’s and she’s in a nursing home. She’s 85. My grandpa is 88 and he’s still going strong.
I never knew my mom’s parents. My dad’s parents are Grandma and Grandpa to all eleven of their grandchildren.
My kid’s grandparents:
My mom and her husband = Nani (or Nonny, depending on who is spelling it) & Grandpa (sometimes Grandpa Alan)
My dad and his wife = Grandpa (sometimes Grandpa Ben) & Grandma
leenmi’s mom and dad (my in-laws) = Nana & Grandpop
my kids call my mom “grandmother” and wife’s mon “Tutu” )Hawaiian for Grandma)
My maternal grandparents are Mimi and Pop*
The paternal grandfather is Grand-Daddy Houston and his second wife we just called by her first name. My maternal grandmother was Mama Billie*.
My maternal great-grandparents were Mammaw* and Pappaw*.
My paternal great-grandparents were Grandmother Icy and Grand-Daddy Prince (Icy and Prince WERE their actual first names. I also have in my family- Princess, Duchess, Prince Jr., but that’s a whole 'nother post)
Now, my children call my mom and dad, Grammy and Grand-Daddy, and Mimi is still Mimi, but Grand-Daddy Houston is now Great-Grand-Daddy Houston.
My eldest two call their paternal grandparents Ma and Pa.
My youngest doesn’t call his paternal grandparents anything, yet, but they prefer Grandma and Grandpa.
FaerieBeth
- = deceased
Dad’s parents: Granma and Granpa
Mom’s parents (both deceased): Grandma Ruth and . . . well, Grandpa Delbert died before I was two, so I probably called him “Kitty” or “Fjord” (my version of fork).
My mother’s parents: Grammy and Pappy*
Bio-Dad’s parents: Gramma and Grampa*
Dad’s parents: Gram and Pap
Dad’s paternal grandmother: Grandma Julia*
Dad’s maternal grandfather: Pap Grimm*
I never knew any of my other great-grandparents.
- = deceased
d_redguy and I don’t have any kids yet, but here are some of our ideas on grandparent names:
My Bio-Dad: Papa (Same thing I call him.)
My Dad: Probably Pap, like his father is to me.
My Mom: Grammy Headcase, because she’s crazy.
d_redguy’s folks: Mr. and Mrs. LastName, because they annoy the snot out of us.
Actually, we’ll probably let the future hyper-reds come up with their own names for the grandfolk- and yes, we’ll make sure they’re respectful ones. 
Both my grandfathers died before I was born, but my two grandmothers were called Granny and Grandma.
Grim
My kids have Granny and Grampa (English) and Baachan and Jiichan (Japanese).
When my oldest boy was a baby he couldn’t say the Japanese names properly and called them Baba and Jiji, which would have been cute except it translates as “stupid old man” and “stupid old woman”! Luckily they thought it was funny, and as soon as he could talk properly he corrected himself naturally.