When we present a genealogy we need a way to uniquely identify each one of the ancestors and a good method is the Sosa numbering system. It is called “Sosa system” because it was made popular in Spain by Jerónimo de Sosa, a Franciscan priest in the XVII century. In the genealogy world is also known as ahnentafel (German for "ancestor table").
The method is simple to use. The first person in the tree gets the number 1. Number 1’s father gets number two and 1’s mother gets number 3. Number 2’s father gets number 4 and his mother gets number 5. Following the same process, number 3’s father gets number 6 and her mother gets number 7.
In summary, the formula for an ancestor n is 2n for the father and 2n+1 for the mother. Therefore, men will always have even numbers and women will have odd numbers, with the exception of number 1 that can be either man or woman.
Knowing the Sosa number it is possible to establish the path between that ancestor and the first perso in the tree using a simple calculation: if it is an even number we know it is man and we divide by two, if it is an odd number we know is a woman so we subtract one and divide by two. This way we obtain the number of the next descendant and we continue the process until we get to the number one.
In addition, knowing the Sosa number we can quickly determine the generation the ancestor belongs and we also know how many ancestors are on each generation. Finally the male line is always represented by the 2 and its multiple numbers: 4, 8, 16, etc.
The numbering system was designed for a person’s direct ancestors, therefore the ancestor’s siblings are not considered. Some alternatives have been proposed, for example if the father is 2, his siblings would be 2.2, 2.2, 2.3, etc. in chronological order, being the oldest brother the first. Other genealogist prefer to utilize letters for the siblings for example if the father is 2, his siblings would be 2a, 2b, 2c, etc. in chronological order. Unfortunately there is not a good way to number cousins and other relatives.