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GEDCOM is an acronym for GEnealogical Data COMmunication. A GEDCOM transmission represents a database in the form of a sequential stream of related records. A record is represented as a sequence of tagged, variable-length lines, arranged in a hierarchy. Each line always contains a hierarchical level number, a tag, and an optional value; a line may also contain a cross-reference identifier or a pointer. Each GEDCOM line is terminated by a carriage return, a line feed character, or any combination of these. The Current version 5.5 defines a universal format that genealogical programs understand. GEDCOM files contain only plain (ASCII) text, and therefore, it can be editted by a plain text editor (not a word processor).
WARNING: manual change of a GEDCOM file may cause logical conflicts!
Even though said to be universal, GEDCOM, is implemented quite differently by genealogy programs. While names, standard events, and relationships usually survive such a transfer, the same cannot be said for notes, sources, citations, user-defined events, name variations, and many other types of data.
Each line of a GEDCOM file starts with a "level" number, which indicates whether the piece of data on that line starts a new record, or is part of the record above. For example, a level "0" line might indicate the start of an "individual" record. The next line might be level "1", and contain the "name" of that individual, as illustarted below:
0 @I13@ INDI 1 NAME Doron /Tal/ 1 SEX M
This is a partial record about an individual whose first name is "Doron", last name is "Tal", gender is Male, and an arbitrary ID of this record is I13.
The true power of the GEDCOM format, however, is that it also contains the relationships between records. For example, further along in the file, we find something like this:
1 FAMS @F13@ 1 FAMC @F11@
It says that this individual is a Spouse in family F13, and a Child in family F11.
Further along, we may find a record that defines each of these families. Family records will contain information such as the IDs of the members of the family, and the date and place of the parents' marriage.
Knowing this basic information, you can now browse a GEDCOM file with confidence.
In a GEDCOM, all the information for each individual is written within a block, but to find out about that individual's spouse, children or parents requires to read the entire GEDCOM file into a database, to restore the pointers and links.