Ahnentafel numbers are a numbering system for showing the relationships in an ancestor tree.  This can best be illustrated by an example.

Suppose we had the following ancestral tree:
 

     John    Jane    Jack    Jill
        |          |          |         |
        +-----+        +-----+
             |                    |
         Donald         Daisy
             |                   |
            + -----------+
                       |
                     me

Note that in the above tree, we always have the husband to the left of the wife.

We start numbering the root person of the tree (me in this case) as 1.  We then number the parents as 2 and 3 by going from left to right across the parent line.  That means the father (Donald) becomes 2 and the mother (Daisy) becomes 3.  Similarly the grandparents are numbered 4, 5, 6, and 7 by going from left to right across the grandparent line.

The rules for generating the Ahnentafel Numbers can be formalized as follows.  The root person is numbered 1.  For any person on the tree, you double his number to get his father's number and you add 1 to his father's number to get his mother's number.

The resulting Ahnentafel Numbers in this case are:

1. me
2. Donald
3. Daisy
4. John
5. Jane
6. Jack
7. Jill

-- Steve Morse