Shorthand ID for Common Ancestors

A Segment-ology TIDBIT

In my spreadsheets, Notes and analyses, I refer to Common Ancestors (CAs), or Most Recent Common Ancestors (MRCAs), by their Ahnentafel numbers.

Most of the time the MRCA with a Match is a couple, and I use the Ahnentafel number of the husband. For example: 36P is my father’s father’s mother’s father’s father (or 1-2-4-9-18-36 with the Anentafel number for each generation). This 36P shorthand actually refers to the 36/37 couple (Thomas NEWLON and wife Susan in my case). I add on a P or M to indicate the Paternal or Maternal side, as this is not obvious with larger Ahnentafel numbers after several generations.

Just to keep my bearings, I also usually indicate the cousinship of a Match – for example: 4C (4th cousin) or 4C1R (4th cousin once removed), or 3Cx2 (double 3rd cousin), or 2C/2 (half 2nd cousin). So the shorthand ID is usually something like 36P/4C1R – a lot of information packed into a compact ID. And, given this shorthand ID, I can always repeatedly divide the Ahnentafel number by two to get back down to me. For example: 856M breaks down to 428-214-107-53-26-13-6-3-1 (me); which is on my mother’s father’s side. I can easily tell that other Matches with 214M and 53M and 13M MRCAs are all on this same ancestral line.


Use a Shorthand ID for CAs and MRCAs

36P/4C1R = the CA is Ahnentafel 36, Paternal side; the Match is a 4th cousin once removed


[22W] Segment-ology: Shorthand ID for Common Ancestors TIDBIT by Jim Bartlett 20190202

19 thoughts on “Shorthand ID for Common Ancestors

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    • Thanks for your feedback. I’m trying to think of a short method for Clusters. Since Clusters tend to form on specific ancestral lines, I’m inclined to use Ahnentafel numbers – as in C126. But we will have several closer Clusters that may be C10 (father’s mother’s father : 2-5-10), but then how to distinguish the C8s apart? C8A, C8B? I expect each C8 to eventually go to C16 or C17. Maybe Clustering at GEDmatch will help coordinate between Companies.


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  6. I like the abbreviated notation but it blurs full or half cousins. I am curious if you have considered this and have a suggestion (I respect your opinion and find this blog very useful).
    I have been using the last names of the parents for full cousins and only the one parent for half cousins. I also include the sibling to indicate the branch when known.

    For example, if my paternal grandfather is Walter, and his brother John is the line for the full match, then I would include their parents Cannon/Bissell>John, along with 2c to show that we are second cousins on my father’s father’s brother’s line.

    If I did the same with ahnentafel numbers, I would get 8/9>John 2c.

    I think that it is useful to distinguish between half and full cousins when using cM predictions. Maybe h2c would work for half second cousins and the default is full.

    If there a numbering system for descendants? (I can’t imagine one that would be useful)


    • Joel, as I indicated, I use a “/2” [divide by 2] to indicate a half cousin. Part of this is just to confirm that I’ve actually worked out the true relationship. Another part is a handy notation in case I want to look up the expected shared cM in Blaine’s Chart at ISOGG. I don’t really sort on this ID, but the /2 explains things if the shared segment seems smaller. Once I have worked out the relationship, this is just a quick way to remember it. Jim


      • Joel,
        Also like you, in my AncestryDNA Notes and my spreadsheet of all atDNA Match-segments, I also list the surnames and the child the Match is from. For example: 84P/4C1R: UNDERWOOD/CANNADAY > Ellinder m RADFORD. If we descend from different wives (or husbands), I use a notation like: NEWLON (1m,2m) to remind me that our shared DNA had to come through the husband NEWLON, and not the wives. These are just shorthand tricks to, so that I don’t have to look each one up again. It’s particularly handy in AncestryDNA Notes carried over into Clusters. Jim


  7. Very interesting notational idea, Jim. I was going to use just a string of F’s and M’ to indicate Father/Mother side all the way up. The only problem I have with the ahnentafel numbers followed by a letter is that in a spreadsheet they sort as a text string and don’t end up in numerical order, e.g. you’ll get: 122P, 12P, 13M, 146M, 1M.


    • Interesting. FMFM notation (does it have a name?) would sort exactly as you want. I don’t like that notation because It is like reading binary numbers and gets confusing. But it clearly marks the branch.
      A solution is to convert the string into a number that drops the p/m and then sort. A similar formula could convert the string into binary and then substitute 1/0s for F/Ms.
      Both would be useful when importing the notes. and the second would make it less necessary to append the p/m that is causing the sorting problem.


      • Here is an excel formula to convert Ahnentafel to MF notation
        =substitute(substitute(right(dec2bin(A1), len(dec2bin(A1))-1),”0″,”F”), “1”, “M”)
        ie 36=MMFMM
        it only works up to 512 or 8 generations, which should be enough for atDNA..


      • Joel, any system (such as I blogged about) can be modified and adjusted to suit each user. My method works for me, and I know of other who use different systems – the bottom line is actually using these tools. Jim


    • Louis – Thanks for your comment. Your insights as an expert atDNA user and programmer are invaluable. In my case, I don’t generally sort on these, I just put them into Notes – like in the AncestryDNA Notes boxes – makes it easier to detect trends in a Shared Match list. Watch for my blog post on using these Shorthand IDs. Jim


  8. If I could wrap my head around the Ahnentafel numbering system I’d use it in my Ancestry match notes. Currently I have a #momdadmom system, the exact relationship (i.e., 2C2R), CA, and whether or not I’ve added the person to my tree #Intree. 🙂


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