Everyone has collateral lines in the family tree. Those are the relatives who are not part of direct lineage, but of the same ancestral line. Such ancestors would be the uncles, aunts, granduncles, second cousins and in-laws of the family.
When completing a family tree it is the direct lineage that is recorded, such as parents, grandparents, great parents, etc. A direct family tree will not show those collateral lines, but they are still very important in the family history.
By doing research on those collateral lines it can open up new areas or sources previously not examined about the direct lineage. For example, if a great-granduncle was found to have serviced in the Canadian military during World War One rather than for the United States military, there was a chance his brother, which would be your great-grandfather might have serviced in that location as well.
Doing detailed research on a relative in the collateral line is very important for not only assisting on the direct lineage, but to provide an overall historical story of the family. You may have learned that it was an older brother of your grandmother who first immigrated to the United States and encouraged his siblings, especially your grandmother to move permanently. It could have been that brother of your grandmother, a great uncle, who even paid for her passage. So many lives were interwoven with their extended family that these collateral lines cannot be overlooked.
There may be a family heirloom that was treasured by everyone over the years. It might have a mark of a date or location and nothing more. If you then found a second cousin of your mother had actually lived in that location around that period, it could have originated from that relative.
What is generally not realized is that all the diverse collateral families eventually merge to share a common ancestor. It may be 2 to 5 generations back, but then the common third great grandparents will be the same for both ancestral lines.
Use the U. S. Federal and the state censuses to see the names of siblings and other relatives in a household at a given time. The more recent censuses placed the relationship to the head of household. If it stated sister-in-law, then the woman listed was more than likely the wife’s sister. If the sister-in-law was unmarried, then there becomes a clue to the wife’s maiden name.
Many of those cousins, in-laws, aunts and uncles generations back experienced some exciting lives. When researching one’s direct lineage gets a little slow or you are stumped, start researching those collateral lines to spice things up.