Cousin relationships explained: first, second, once removed
Quick answer: Find your closest common ancestor. Shared grandparents make you first cousins, shared great-grandparents second cousins. If one of you sits a generation lower, add "once removed" for each generation of difference.
"Second cousin once removed" sounds like a legal term, and most people nod politely without knowing what it means. It is actually a precise description of a family position, and once you know the counting rule you can work out any relationship in about ten seconds.
This matters beyond family reunions. If you take a DNA test, your match list is full of predicted cousin relationships, and reading them correctly is what turns a match into a place on your tree.
The one rule that does the work
Ignore the labels at first and find the closest common ancestor — the person or couple you both descend from.
Now count generations from that ancestor down to each of you:
- Equal distance? You are on the same cousin level.
- Unequal distance? The smaller number sets the cousin level, and the difference becomes the "removed" part.
Example: your mother's first cousin descends from your great-grandparents, two generations down. You descend from the same great-grandparents, three generations down. The smaller number gives "first cousin", the one-generation gap gives "once removed".
The cousin chart
| Closest shared ancestor | Relationship | Example |
|---|---|---|
| Parents | Siblings | brother, sister |
| Grandparents | First cousins | your aunt's child |
| Great-grandparents | Second cousins | your grandmother's brother's grandchild |
| Great-great-grandparents | Third cousins | one more generation back |
The word "removed" never describes the gap between the cousins' ages — only the gap in generations. Two people born in the same year can easily be first cousins twice removed if one branch of the family had children early.
So: your first cousin's child is your first cousin once removed. Your first cousin's grandchild is your first cousin twice removed. Your grandparent's first cousin is also your first cousin twice removed, seen from the other direction.
Great-aunts, grand-aunts and the "great" muddle
Your parent's sibling is your aunt or uncle. Your grandparent's sibling is your great-aunt or great-uncle — some genealogists prefer grand-aunt and grand-uncle, because the person sits on the grandparent generation. Both are in common use and both are understood; pick one and stay consistent inside your own tree.
Looking down the tree, your sibling's child is your niece or nephew, and their child is your great-niece or great-nephew.
One warning for record work: "cousin" in older letters, wills and church records was often used loosely for almost any relative, including in-laws and neighbours' kin. Treat the word as a hint, never as proof of a relationship.
Half, step and adoptive relationships
These three are frequently mixed up, although they are quite different:
- Half-siblings share one parent. They are biologically related, and on average share about a quarter of their DNA.
- Step-siblings share no parent at all — their parents married or partnered. There is no biological relationship.
- Adopted children are legally full members of their adoptive family. The biological line still exists and is a second research line, not a replacement.
The same distinction applies to in-laws. Your aunt's husband may have been "Uncle Bob" all your life, but he is related by marriage, not descent. Keep both facts in the tree: the relationship people lived, and the descent that records can prove.
Double cousins and other special cases
Occasionally two siblings from one family marry two siblings from another. Their children are double first cousins: they share both sets of grandparents instead of one, and they share roughly twice as much DNA as ordinary first cousins — about the same as half-siblings.
Cousin marriage itself is a normal part of many family trees, especially in small rural communities before the twentieth century. Whether first cousins may marry today varies widely: it is legal across much of Europe, and in the United States it depends on the state. If it matters for a living family, check the current law where they live rather than relying on a general rule.
Why this matters for DNA matches
Testing companies estimate relationships from the amount of shared DNA, and those estimates come with wide ranges. Averages are roughly 12.5 percent for first cousins, 3 percent for second cousins and under 1 percent for third cousins — but actual results vary a lot, and beyond third cousins two genuine relatives may share no detectable DNA at all.
That is why a match list is a starting point, not an answer. Use the counting rule above on documented ancestors, and see which cousin level would fit the shared amount.
How to work out any relationship
- Find the closest common ancestor
- Count generations down to each person
- The smaller count sets the cousin level
- The difference becomes "once/twice removed"
- Check whether it is descent or marriage
- Note half, step or adoptive relationships explicitly
- Compare with DNA estimates only as a cross-check
Next step
Relationships are far easier to see than to describe. Put the common ancestors into a tree and count the generations down each branch — siblings and partners follow automatically from the structure: Create a free family tree.
Read next
- Starting genealogy: first steps
- DNA testing for genealogy
- Create a family tree with children and grandchildren
- Printable ancestor chart
- Family tree school project
FAQ
What is a second cousin?
Someone who shares your great-grandparents — the grandchild of one of your grandparents' siblings. First cousins share grandparents instead.
What does "once removed" mean?
That the two people are a different number of generations from the shared ancestor. Your first cousin's child is your first cousin once removed.
Is my cousin's child my second cousin?
No, that is the most common mix-up. Your cousin's child is your first cousin once removed. Second cousins are on your own generation level.
What is the difference between a great-aunt and a grand-aunt?
Nothing — both describe your grandparent's sister. "Great-aunt" is more common in everyday speech, "grand-aunt" is preferred by some genealogists.
How much DNA do cousins share?
On average about 12.5 percent for first cousins and 3 percent for second cousins, with wide variation. Third cousins and beyond may share little or nothing, so DNA alone cannot fix the exact relationship.