Well, great-grandparents are great-grandfather and great-grandmother.
– the father of your grandfather
– the grandfather of your father
– and the husband of your great-grandmother.
– the mother of your grandmother
– the grandmother of your mother
– and the wife of your great-grandfather.
Based again on the sample of the British royal family, let’s see the blood kinship of the different members of the royal family from the view and angle of the great-grandparents. Check the illustration below!