An ahnentafel is an "ancestor table." It lists all known ancestors for the first person listed - person number one. The father for any named person has the number equal to the person's number times two. The mother's number is the person's number times two plus one. Therefore, a man's wife has a number one greater than his own. For instance, the father of person 10 is person 20 and 10's mother is 21. The mother's father, then, is person number 42... An interesting thing happens if you convert an ahnentafel number to binary - Each digit represents the number of generations of descent and the number itself describes, in gender, the lineage to the given person. For instance, 20 in binary notation is 10100. -- Five generations. Person number one is always at the left and is always represented by a one, regardless of that person's gender. However, moving to the right, you move up the tree by gender, with 0 representing a male and 1 representing a female. Person number 20, then, is a paternal great-great grandfather of person number 1.
Example
First Generation
1 (1): me
Second Generation
2 (10): my father
3 (11): my mother
Third Generation
4 (100): my father's father
5 (101):my father's mother
6 (110): my mother's father
7 (111): my mother's mother
NOTE: The first number is the decimal and the number in (perenthesis) is that number's binary equivalent. So, since the first number is always me, all binary annotations start with '1'. If you look at the number '7', you see it's binary equivalent is '111'. This means that the person represented by 7 (or 111), is the mother of the mother of the first person - of the first person's maternal grandmother. Remember, 1's always denote females (save for the first person), and 0's always denote males.
Related Node: Genealogy