In family relationships, grand- is used for one's parents' parents and for one's children's children, while great- is used for one's grandparents' parents and one's grandchildren's children: My grandfather was born in London and his father, my great-grandfather, in Reading. For relatives by marriage one can use great- or grand-, but great- is always more common: He had a surprise visit from his great-niece; She has kept in touch with her grand-aunt.