The Henry system is a numbering system used for cataloging descendants of one person. Brainchild of Reginald Buchanan Henry, the Henry system assigns the number 1 to the individual who starts the line of descent, similar to other numbering systems. The difference comes at the second generation, where the oldest child of person number 1 is given the number 11, the second child the number 12, and so on. In the third generation, the oldest child of person 11 is assigned the number 111, the second child of that person is 112, and so on. Number 12's oldest child is 121, the second child 122, and so on.
You can see that the number of digits indicates the generation being referenced: a one-digit number is a person in the first generation, while a four-digit number is assigned to each person in the fourth generation. Another advantage is that the descendants of more than one progenitor may be tracked. For instance, three immigrants to Massachusetts in the mid-1600s had the last name of Dow. No relationship has ever been proven among these three men. However, the descendants of each may be traced within one report. One immigrant is given the number one, a second is assigned number two, and the final immigrant is listed as number three. Third-generation descendants might have the numbers 123, 267 or 322. In each case, it is easy to see the progenitor of each line by looking at the first digit of any person’s Henry number.
The disadvantage of Henry numbers is that these numbers quickly become large as the number of generations increases. Also, families with more than nine children require other means of numbering. Many researchers assign the letter A to the tenth child, B to the eleventh, and so on. This solves the problem by allowing for 35 children per family, surely enough for most of us.
Another method of dealing with more than nine children in a family is to use parentheses: 3(12)4, for example.
d'Aboville Numbers are almost the same as Henry Numbers, except that each digit (or double digit for numbers larger than 9) is separated by a period.