FamilySearch publishes the GEDCOM standard. It also refuses to let you export its family tree to GEDCOM, and its help centre is blunt about it: "You cannot download GEDCOM files from Family Tree." The reason given is the single best description of the format anyone has written. "The version of the GEDCOM standard that is most widely used does not provide a way to store reason statements and other types of information that are crucial and unique to Family Tree."
The body that owns the spec will not trust its own data to it. That is worth knowing before you move a tree you have spent years on.
None of what follows means GEDCOM is useless. It moves the skeleton, meaning names, dates and who belongs to whom, and it moves it reliably. It is the connective tissue that falls off.
The file is plain text, so your photos were never inside it.
This catches people every time. A GEDCOM is a text file, and a text file cannot contain a JPEG. What it can contain is a path pointing at where the picture sat on the machine that made the export. MyHeritage's help page puts it plainly: "A GEDCOM file is a text file, that doesn't include the actual photos but it may contain the links to the photos original location on your computer or family site."
Move the file to a new computer and every one of those paths points at nothing. If your media matters, download it separately and keep it beside the file. The same page reports that Ancestry has begun blocking outside services from pulling media across, which is MyHeritage describing a competitor rather than Ancestry stating its own policy, so treat it as one vendor's account. The practical effect for you is the same either way.
Every program stores things the format has no word for.
Gramps documents this against itself, which is rarer and more useful than any third-party comparison. Its own wiki says GEDCOM export is "necessarily a lossy operation" and then itemises the casualties. "Call Name: no equivalent in GEDCOM." "Source Attributes (previously Data): no equivalent in GEDCOM." "One gender on same sex relation: GEDCOM does not support same sex relation." Note markup goes. The place hierarchy introduced in Gramps 4.1 has no GEDCOM structure to live in. Event ordering is ignored.
Its download page draws the obvious conclusion and tells users never to archive in the format: "GEDCOM cannot store all the intricacies found in Gramps data." One caveat on that list, in fairness to Gramps: the page anchors itself to a much older release, so read it as the shape of the problem rather than a line-by-line audit of the current version.
Family Tree Maker publishes the list, and it is longer than you expect.
MacKiev documents what its Ancestry sync leaves behind in both directions. Going up to Ancestry: research notes, fact notes, media notes, relationship notes, text formatting, media categories, audio and video items, GPS coordinates, colour coding, saved charts and reports, and the change log. Coming down from Ancestry: comments, stories, albums, web links, photo tags, and DNA results.
Their own warning is the sentence to remember, because people treat a synced online tree as a safety net: "it is important not to regard your Ancestry tree as a full backup of your FTM tree." That is a sync between two products from companies that work together. A GEDCOM between two unrelated programs has fewer guarantees, not more.
Unrecognised data sometimes survives, in a file nobody opens.
When the spec has no tag for something, vendors invent one, by convention prefixed with an underscore. The receiving program then meets a tag it has never heard of and has to decide what to do. RootsMagic documents its answer: "If RootsMagic encounters any unfamiliar data in the GEDCOM file, it puts that data in a 'list file' on your disk in the same directory as the new database."
So the data is not always destroyed. It is set aside in a sidecar text file, next to your new database, which almost nobody ever opens. If you have migrated and something feels missing, that file is the first place to look, and it is the strongest argument for checking an import rather than assuming it.
The version everyone uses spent twenty years as a draft.
The history explains the mess. The 7.0 specification records that the previous major version, 5.5.1, "was released in draft status in November 1999 and re-released as a standard in October 2019". The format the entire industry has exchanged data in since the 1990s was not a ratified standard until 2019, which is exactly how you end up with dialects.
Version 7.0 arrived in 2021 and fixed real things, including character encoding: "Previous versions allowed multiple character encodings, defaulting to ANSEL. 7.0 only uses the UTF-8 character encoding." It also broke compatibility on purpose. The spec states that "5.5.1 files are, in general, not valid 7.0 files and vice versa".
Whether a program reads version 7 is often not written down where you would look.
This is the part that makes planning hard, and we can only report the pages we checked. Synium says it outright on its tech specs page for MacFamilyTree: "Support for GEDCOM 5.5.1 as well as GEDCOM 7". Reunion 14's feature list names GEDCOM 7.0 and GEDZIP. Others are quieter. The RootsMagic import page quoted above names no version at all, and neither does the MyHeritage article above it, so for those two the answer is not on the page a searcher would land on. Where a vendor does not say, you are left to find out by exporting and looking.
Four things to do before you move anything.
First, export your program's own native backup alongside the GEDCOM, because both Gramps and MacKiev tell you to in their own words. Second, download your media separately, since it was never in the file. Third, import into the new program and compare counts of people, families and sources against the old one before you delete anything. Fourth, keep the original file and the original program installed until you are certain, which usually takes longer than one evening.
Our GEDCOM viewer will open the file in your browser and show you what is actually in it, which is the cheapest version of that third step. The guide to opening a GEDCOM file covers the format itself, our post on Gramps review covers the program that documents its own losses most honestly, and our genealogy software comparison covers where each program stands.
The uncomfortable summary is that the interchange format everyone relies on is a lowest common denominator, and every program worth using stores more than it can express. Plan the move around that and you will keep almost everything. Assume the file is a backup and you will find out which parts were not, months later, when the original is gone.
