Second Cousins, Once Removed: Relationship Terms Made Simple

What "first cousin once removed" actually means, how to work out any relationship in two steps, and why it matters for DNA matches.

2 min read

Cousin terms confuse almost everyone, but there are only two rules to learn. Once you know them, you can name any relationship in your tree, and you'll understand what DNA testing sites mean when they call someone your "third cousin."

Rule 1: The cousin number comes from your shared ancestors

Cousins descend from the same ancestors through different children. The number tells you how far back that shared couple is:

A quick way to remember it: the cousin number is the number of "greats" plus one. Or count the generations back to the shared couple and subtract one.

Rule 2: "Removed" means a difference in generations

When two people aren't in the same generation, the relationship is "removed." Each generation of difference is one removal.

• Your first cousin's child is your first cousin once removed. You're one generation apart.
• Your first cousin's grandchild is your first cousin twice removed.
• Your parent's first cousin is also your first cousin once removed. Removal works in both directions.

Your first cousin's child is not your second cousin. That's the most common mistake. Your children and your first cousin's children are second cousins to each other, because they share great-grandparents.

Working out any relationship in two steps

1. Find the shared ancestor and count how many generations each person is below them.
2. Use the person who is closer to the ancestor to get the cousin number (Rule 1), then count the difference in generations for the removal (Rule 2).

Example: you are 3 generations below your great-grandparents. Your mother's cousin Linda is 2 generations below them (they're her grandparents). The closer person, Linda, makes it first cousins; the difference is one generation. Linda is your first cousin once removed.

Why this matters for DNA

DNA testing sites estimate relationships from how much DNA you share, measured in centimorgans (cM). Roughly, first cousins share an average of about 870 cM, second cousins about 230 cM, and third cousins about 75 cM, though the real numbers vary widely. Different relationships can share similar amounts. A first cousin once removed and a half first cousin share about the same, for example. Knowing the terms helps you read those predictions. For more, see Using DNA Matches to Break Through Brick Walls.

In-laws and "steps"

Relationships by marriage don't follow cousin math. Your spouse's cousin is not your cousin, and a step-parent's relatives are related to you by marriage, not by blood. In your tree, record these through the marriage that connects them.