Ahnentafel

Ahnentafel (German for “ancestor table”) is a numbering system used to systematically list a person’s ancestors. In this system, the subject is numbered 1, their father is 2, their mother is 3, paternal grandfather is 4, paternal grandmother is 5, and so on.

The pattern follows a simple rule: a person’s father is always twice their number, and their mother is twice their number plus one. This allows any ancestor’s number to reveal their relationship to the subject without needing to trace through the chart.

The system is also known as the Sosa-Stradonitz system, named after the Spanish genealogist Jerónimo de Sosa (1676) and German genealogist Stephan Kekulé von Stradonitz (1898).