I called my grandparents Grama and Granpa (last name). Although I never knew my fathers dad. We couldn't say just the last initial because they were the same letter.

My nieces call my parents Grama and Granpa and my SILs parents Nana and Papa. They called their stepgrandma Grama Billie, but she is no longer living. I'm actually not sure what they call their Nana's husband. I think just by his first name.

Bruce's nephew calls his mom Grammie and the other Grandmother Gran B. B for her last name. And like Cataholic said, they chose the names they wanted him to call them.