We commonly think of collateral as a loan or damage. Genealogists use the term to define a blood relative that is not a direct ancestor.
A direct ancestor is your parent, grandparent, great-grandparent and so on, in a family line. A blood relative is not in your direct line — they are an aunt or uncle, cousin, nephew or niece, etc. But at some point all the diverse collateral families eventually merge to share a common ancestor. It may be several generations back, but then the common great-grandparents (2x, 3x, 4x, etc.) will be the same for both ancestral lines.
If you are adrift in the search for an ancestor, the collateral relative may be the key that opens the elusive door. A family tree shows the direct family line but does not show the collateral lines, even though they are important in the history of the family.
How often do we learn that an ancestor came to America to join a sibling, parent, or relative? And they sometimes followed a friend, associate, or neighbor? Researchers refer to them as the FAN Club. Friend, Associate Neighbor.
Immigrants usually lived in proximity to one another when arriving, like my great-great-grandparents from Wurttermberg, Germany. They came to America in the late 1840s and settled in Cincinnati, Ohio in an area called Over-The-Rhine. Looking at the census with my grandparents listed, the entire page is people from the same area of Germany.
Studying history you might find an epidemic, religious persecution, political upheaval, famine, poverty from lack of work, social disparity, etc. that explains large numbers immigrating from Europe in a particular year.
Collateral lines can reveal sources that you may not have considered. An example would be: Your family story is that your great-grandfather from New York state served in WWI but you don’t know where and your can’t find military records for him. Fold3 is a good resource for military records, but not all of them are there. You find his brother’s service record but it shows him serving in Canada, not the United States. There is a clue: check Canadian military records, as both brothers may have served in Canada.
In census records, take note of the household aside from parents and children — boarders, servants, and unspecified members. They might be family.
Don’t overlook your own collateral relatives. They could have family heirlooms, documents, and more that will help your research
A personal collateral relative story: A few years ago I decided to join the Society of Civil War Families of Indiana using my 2x great-grandmother’s brother, a veteran who fought at Gettysburg. I had documents. But I learned that I have to be a direct descendent and I am not, only a blood relative. That is sad, because he never married nor had children; his line ended with him.
I think of collateral research as taking a straight road, then branching off the road to find a surprise waiting.
“Genealogy is learning about the time of your ancestors, not just names and dates.”