by Richard A. Pence

Sosa-Stradonitz System OR Ahnentafel

Copyright 1995 by Richard A. Pence - Posted with Permission

The normal - and extremely easy and effective - method of numbering your ancestors is to assign yourself (or child) the number 1. If you are No. 1, then your father is No. 2, your mother No. 3, your paternal grandfather No. 4, etc. In this system, a person's father's number is always twice the person's number and his or her mother's number is twice plus one. This method of numbering one's ancestors is used worldwide and is called the Sosa-Stradonitz System for the Spanish genealogist Jerome de Sosa who first used it in 1676 and for Stephan Kekule von Stradonitz who popularized it in his 1896 Ahnentafel Atlas. It is also sometimes called the "Ahnentafel Numbering System," after the book. (In popular usage today, an ahnentafel is a listing rather than a chart of ancestors.)

If you want to maintain information on collateral relatives in your database, you can get a unique identification number for any such relative in any line of descent by using the Sosa-Stradonitz number of the common ancestor, followed by a decimal point and an expansible descent number based on the Modified Henry System (see COMBINED NUMBERING SYSTEMS later).

Advantages and Disadvantages

Whether you use a computer or not, there is really no substitute for the ahnentafel numbering system. It is widely accepted and understood - so widely accepted that it is almost universally used. It also has the virtue of being mathematically uniform and, therefore, is made-to-order for computer use. A computer can be easily programmed to find parents, grandparents, great grandparents, etc., by use of the prevailing rule of the ahnentafel: The father's number is always twice as large as the child's and the mother's is one greater than that. That one fact makes tracing descent back or down a snap for a computer. It also makes it possible to program a computer to print charts for any person in the genealogical data base and use a different numbering sequence each time. For instance, you might want to print out a set of ancestor charts for someone else using your grandfather as the subject (No. 1). The subsequent numbering of and within the charts can be easily accomplished.

The ahnentafel system allows almost limitless additions. However, two problems can arise:

1. The same person can pop into your family tree a second time, thus requiring two different records and two different numbers; the thing to watch for here is to make certain that both records contain the (same) correct information. A National Genealogical Society Computer Interest Group DIGEST reader noted that the Stradonitz system "allows extension ad infinitum, but computer problems arise when two different lines lead to the same person and slavish adherence would produce duplicate data under different numbers, or varying data if the research is done twice from different sources and the genealogist fails to go back to the primary source...."
2. The same arithmetic progression that makes the ahnentafel wonderful for keeping track of ancestors could become a problem if you are fortunate to trace back many generations. The space allowed for the ahnentafel number in your database could tax a small computer. However, with six digits you can handle 19 generations of ancestors, so as a practical matter most of us don't have to worry. On the other hand, a random-access database which relies on there being a record for each individual in an ahnentafel would fill up a disk in just five generations - and half those records might be blanks. (This can be programmed around with a little foresight.)