Rev. H.O. Sheldon devised a unique numbering system for Sheldons. He explained this system in the "Preface" to Volume I:
Every descendent has a number. The numbers are all placed mathematically; 100 upon each double page. The last number by cutting off the 00, being the number of the page: so that all the numbers, except the last, will be found upon the page preceding the hundreds in the number; as, all below 100 on the first page; above 500 and below 600 on the sixth page. Hence 01 with the hundreds will always be found on the top of the left side; 25 at the middle, 50 at the bottom, 51 at the top right side, 75 at the middle. To facilitate tracing descendants, the reference designates the side of page; reference to the right side having r added. A * instead of a reference figure, denotes descendants whose names we have.
Loudonville, Ohio, June 7th, 1855
S#5297×6-2, Carew Sheldon, added to this explanation in Volume 5 to show how Sheldons are numbered who are not listed in the original Sheldon Magazine:
As described in the original Preface of the 1855 which is reprinted in the section of Indexes, the S#s in Vol. V indicate the page and line where the Sheldon head-of-family can be found in Vols. I to IV as a child along with his parents and other data including a cross-reference to Vol. V. As an example, S#5297 will be found on page 53 on the 47th line of the right-hand page, since they are numbered in duplicate with 50 lines to a page. There were 11,250 S#s for that many possible families as of 1855. No higher S# will be used.
Starting with the S# of the last ancestor shown in Vols. I to IV, all of his descendants will always take that family S# plus a line-of-descent or extension number to be shown as S#_x_. The extension # or "x" part is merely the order of birth in each following generation, hence everybody should be able to supply 3 or 4 of their own "x" numbers and future generations can do likewise, thus:
S#5297×1 is the first unlisted child; and the 9th is S#5297×9. If full order of birth in a generation in unknown, leave blank spaces S#5297x- -2-1. If last printed ancestor is unknown, leave blank spaces S#____x- -2-1. This numbering system starts with Volume V and should be used in all Extensions, on Sheldon Family Data Cards in the places provided, wherever any Sheldon is mentioned including signatures to letters.
Since many have the same name, this will facilitate searching and filing of the cards which are arranged by S# x – -. Most of the data with indexes has now been printed up to 1850 to help you find your ancestor.
Thus, numbers preceding the "x" show the last person in the original magazines including the descendant's page and line position. Numbers following the "x" indicate generations after the last original entry and the sequence of birth of the descendant within a specific generation.
Today, we often see the pound sign (#) replaced with a hyphen (–), as in S-5297,or no character at all between the "S" and the first digit of the Sheldon number, as in S5297. The use of the hyphen between generations, as in S5297x-3-2-1, is also not commonly used, rather the birth order of the generations is run together, as in S5297x321. If the birth order exceeds 9, for this example let's assume we want to indicate the eleventh birth child, the digits are placed between hyphens, as in S5297x321(11). Adopted descendants are indicated with the letter "A".
When the Sheldon Family Association began to convert its records and files to a computer format, a new numbering system was necessary.S9759x71 E. Mark Sheldon, the author and developer of the Sheldon Family Genealogy System and great-grandson of Rev. H. O. Sheldon, explained his computer numbering system in the introduction of his four volume Master Index Set:
This number is unique to the Sheldon Family Genealogy System and compatible with the numbering system developed by Keith M. Sheldon for his book John of Providence. The computer number is in the form of an alpha character followed by a series of the numbers or letters. The initial alpha letter will be a "G", "I", "J", "R" or "W" and indicates the Sheldon line of descendant as noted in the table below. Each subsequent letter or number represents a generation and the birth order in that generation for purposes of this system. Since a generation is represented by a single character, numerical characters would limit family size to 9 children. To alleviate this problem, capital letters "A" through "V" are used to represent the tenth through the thirty-first child. To date  the largest family has been 23 children.
S0004 Godfrey Sheldon G1
S0008 John of Providence J1
S0005 Isaac Sheldon I1
S0013 John of South Kingston W1
S0022 Richard Sheldon R1
In 1988, The Federation of Genealogical Societies presented its Award of Merit to E. Mark Sheldon in recognition of his Sheldon Family Genealogy System noting that while there were numerous programs written for the individual genealogist, there were none available for the group genealogy club or association. All Sheldons are deeply indebted to Mark for the thousands and thousands of hours he has tirelessly devoted to the Sheldon Family Genealogy System and the Sheldon Family Association.
The above article was prepared by S#4727×514 Wayne E. Nelson from information in The Sheldon Magazine and other Sheldon sources.