Genealogical Numbering Systems Explained: Ahnentafel, Register, d'Aboville and Henry

How genealogists number people in books and charts: the Ahnentafel system for ancestors, and the Register, NGSQ, d'Aboville and Henry systems for descendants.

Open any printed genealogy and you will see numbers next to the names. They are not page numbers or dates. They are a numbering system, and each system encodes the relationship between people so that a reader can find their way through hundreds of pages without a diagram. Here are the ones you will meet.

Ahnentafel (ancestor table)

The Ahnentafel is the standard for ancestors. The starting person is number 1. A person's father is twice their number and their mother is twice their number plus one:

• 1: you
• 2 and 3: father and mother
• 4 and 5: paternal grandparents; 6 and 7: maternal grandparents
• 8 to 15: great-grandparents

Every man has an even number and every woman an odd number, except for the starting person. Because the numbers double with each generation, you can tell at a glance that number 37 is in the sixth generation (numbers 32 to 63) and that she is the mother of number 18. The system was published by Michael Eytzinger in 1590 and is still used unchanged.

Register system

The Register system, used by the New England Historical and Genealogical Register since 1870, numbers descendants. The progenitor is 1. Children are listed under the parent with lower-case Roman numerals (i, ii, iii); only children who have descendants of their own receive an Arabic number and their own entry later in the book. Superscript numbers after names show the generation.

NGSQ (Modified Register) system

The National Genealogical Society Quarterly system works like the Register system, but every child receives an Arabic number, whether or not they are carried forward, and a plus sign marks children who have their own entry. It is easier for readers who want to see every person numbered.

d'Aboville system

Popular in France and continental Europe, the d'Aboville system builds the number from the line of descent: the progenitor is 1, their children are 1.1, 1.2 and 1.3, the second child's children are 1.2.1 and 1.2.2, and so on. The number is the path through the tree. It becomes long after ten generations but never ambiguous.

Henry system

The Henry system compresses the same idea into a single string: the progenitor is 1, children are 11, 12, 13, grandchildren through the second child are 121, 122. When a family has more than nine children, letters or parentheses are used for the tenth and beyond.

Which system should you use in a book?

• An ancestor book uses the Ahnentafel; there is no serious alternative.
• A descendant book in the English-speaking tradition uses the Register or NGSQ system.
• A descendant book for continental readers often uses d'Aboville, and the path-like numbers double as an index.

Whatever you choose, apply it consistently and print a short explanation at the start of the book. Books generated by PrintFamilyTree number people automatically from the structure of your GEDCOM file, place the numbers next to the names on the charts, and generate the index so that every person can be found by number or by name.