Wednesday, July 20, 2011

Calculating Ahnentafel Numbers

Genealogical numbering standards are something I'm peripherally aware of. I know they exist, I know my software can display or output them in various ways, but I don't really use any numbering systems in my day to day genealogical work. So how did I get to calculating Ahnentafel numbers?

Randy Seaver of Genea-Musings, in last week's Saturday Night Genealogy Fun, asked us to list our 16 great-great grandparents. Fine - this I can do. But I noticed in Randy's post that his 16 great-great grandparents were numbered from 16 to 31. I figured this numbering was a standard of some kind, so I turned to Google, and then found myself in Wikipedia.

Wikipedia's article on Genealogical Numbering Systems helped me determine that the numbering in Randy's post was the ahnentafel system. I thought the Wikipedia article on ahnentafel was quite good, and in addition to explaining the system, explained a couple of ways to calculate the numbers. So I decided to try and calculate the ahnentafel numbers for my 16 great-great grandparents. Sure, Reunion could spit these out for me, but I liked the idea of doing the calculations and as part of that, understanding better the basis of the system itself.

I chose to calculate my numbers using Wikipedia's "second method," explained in the Ahnentafel article, using binary to represent each father ("0") or mother ("1") in the relation string, and then converting the binary number to a decimal. I worked from my father's paternal line to my mother's maternal line, and the results were:

Sara's father's father's father's father
10000 = 16
Richard Cranch Greenleaf (b.1845)

Sara's father's father's father's mother
10001 = 17
Adeline Emma Stone

Sara's father's father's mother's father
10010 = 18
William Adams (b.1840)

Sara's father's father's mother's mother
10011 = 19
Helen Coolidge

Sara's father's mother's father's father
10100 = 20
Thomas Joseph McCormick

Sara's father's mother's father's mother
10101 = 21
Margaret Gilligan

Sara's father's mother's mother's father
10110 = 22
Henry Edward Gillespie

Sara's father's mother's mother's mother
10111 = 23
Anna Ambrose

Sara's mother's father's father's father
11000 = 24
Petera Brown Worrall

Sara's mother's father's father's mother
11001 = 25
Mary Catherine Young

Sara's mother's father's mother's father
11010 = 26
Joseph Harrison Collins

Sara's mother's father's mother's mother
11011 = 27
Rebecca Sharpless Delany

Sara's mother's mother's father's father
11100 = 28
Richard McCall Cadwalader

Sara's mother's mother's father's mother
11101 = 29
Christine Williams Biddle

Sara's mother's mother's mother's father
11110 = 30
John Ruckman Fell

Sara's mother's mother's mother's mother
11111 = 31
Sarah Rozet Drexel

I used this tool to covert the binary numbers to decimal.

It took an evening to do, but was fun and productive genealogically in a new way.


  1. Well done! I wondered if anyone would catch the numbers I used. I didn't think about it until after I posted it.

    Part of the Fun of SNGF is trying out new things and learning new things.

    Your Binary number use is interesting - something new for me. Thanks!

  2. This is best explanation of ahnentafel numbers I've ever read.
    It shows the binary numbers inside an ahnentafel!

    Thomas M.