Grandparent
A person's ancestor tree is a binary tree. It is formed by the person, the parents (2), the grandparents (4), great-grandparents (8), and so on. With each increasing generation of great-grandparents the number doubles. In other words, a person's sixth great-grandparents (great-great-great-great-great-great-grandparents) would number 256 or  (2 to the 8th power). However, the number of individuals in such a tree grows exponentially and will eventually become impossibly high. For example, a single individual alive today would, over 30 generations going back to the High Middle Ages, have about  generations or just over a billion ancestors. That would be far more than the total world population at the time.