For a father's father:
* Koro: This is the most common term, used by both males and females.
* Tūpuna: This term is more general and means "ancestor" or "forefather," but can also be used for grandpa.
* Papa: This is less common but is sometimes used for grandpa, especially by younger people.
For a mother's father:
* Tūpuna: This is the most common term, as it's more general.
* Koro: This can be used, but it's not as common.
Other considerations:
* Ngāpō: This term refers to a grandfather on either side of the family. It's less common than "koro" and "tūpuna."
* Grandfather's name: Many people refer to their grandfather by their name instead of using a general term.
It's always best to ask the person directly how they prefer to be addressed, as there may be personal preference or family tradition that dictates the best term to use.