I'm Granny - the same as my mother was to her grandchildren and her mother was to me.  The grandchildren's other grandmothers are Nanna or Nanny.

When I was a child I had a Granny and a Grandma; I always thought that Grandma sounded more old fashioned.

My husband is Grandad to the four who don't have another grandfather and Grandad Terry to the boy who does have two grandfathers, his other grandfather is Grandad Eric.