Your mother's cousin's daughter is your second cousin

How It Is Counted

Wright's fraction, the Church's count, the courts'

The Name

Any blood relationship between two people is two climbs to the nearest ancestor they share: a generations up from one, b up from the other. The name follows from the two counts. With one climb on either side the pair are siblings, aunts and uncles, or nieces and nephews; with two or more on both sides they are cousins. A cousin's degree is the shorter climb less one, and the removal is the difference between the climbs. A mother's first cousin and a first cousin's son are both first cousins once removed: one generation away, one above and one below.

Two conventions name the rungs beside a direct line. Common usage adds great- for every generation past the first, so a grandparent's sister is a great-aunt; the genealogical convention keeps the rung parallel with the grandparent and calls her a grandaunt. The page prints both, and from three greats on it writes the count: 3rd great-grandmother. Half relatives descend from one shared parent and two different partners. The gender-neutral forms are pibling for a parent's sibling and nibling for a sibling's child.

The Fraction

Sewall Wright defined the coefficient of relationship in 1922 as a sum over every path that joins two people through a shared ancestor, each path worth one half for every link in it, in a family with no cousin marriages. Full relatives descend from a couple, so two paths join them, one through each partner; half relatives and the direct line have one.

full: r = 2 × (½)^a+bhalf or direct: r = (½)^a+b

Second cousins are joined by two paths of six links: 2 × (½)⁶ = 1/32. The page never prints a percent. Each generation doubles the denominator, and a percent hides the doubling and rounds it away.

The Shared DNA

A parent passes on one full copy of the autosomes, about 3,400 cM of a 6,800 cM total, the figure the ISOGG Wiki and the Shared cM Project use for their expected values. The expected match a DNA test reports is the coefficient times 6,800 cM, except for full siblings, who can match on both copies at once: three quarters of 3,400, or 2,550 cM. The observed range is the Shared cM Project 4.0 (Blaine T. Bettinger, March 2020), built from submissions for nearly 60,000 known relationships, with the highest and lowest 1 in 200 set aside from each range. Testing companies use other totals (23andMe 7,440 cM, by DNA Painter's note), so a reported match can sit above the expectation with nothing wrong.

The Two Degrees

Civil law counts every generation up to the shared ancestor and down again, so second cousins stand in the sixth degree. The older canon-law count, the 1917 Code's canon 96, counts only the longer line: second cousins in the third degree. The 1983 Code of Canon Law, canon 108, counts the collateral line the civil way, “as many degrees as there are persons in both the lines together, not counting the common ancestor.” The page prints both counts and reads no law: whether a statute turns on a degree is a question for that statute.

What It Will Not Count

• Marriage, step-family, in-laws, adoption, fostering and godparents. They are family, and they are not shared descent; the phrase reader names them and stops.
• A step that could leave the blood line. A cousin's mother may be an aunt by blood or by marriage, and the words do not say which, so the page asks for the line instead of guessing.
• Double relationships. Double first cousins share two pairs of grandparents and twice the coefficient; the page counts one fork.
• Cousin marriage, endogamy and pedigree collapse. Real sharing runs above the expectation, and the page cannot see a family's history.
• Parentage. The DNA figures are averages across many families, never a test of one.
• Other languages. Many split relatives the English words merge; the page names relatives in English only.

Receipts

The widest gap is in the data, not the arithmetic: observed averages for distant cousins run above the expectation, by about a sixth for half first cousins twice removed: 125 cM against 106. The page prints both numbers and adjusts neither.

Sources

• S. Wright, “Coefficients of Inbreeding and Relationship,” The American Naturalist 56 (645): 330–338, 1922. doi:10.1086/279872
• B. T. Bettinger, The Shared cM Project, Version 4.0, March 2020, and the chart's interactive edition by Jonny Perl at DNA Painter. Used with attribution under CC BY-NC-SA 4.0, the stricter of the two licences the project's pages state.
• Code of Canon Law (1983), canon 108.
• J. T. Burtsell, “Consanguinity (in Canon Law),” The Catholic Encyclopedia, vol. 4, 1908.
• W. Clarke, Table of Consanguinity showing degrees of relationship, Wikimedia Commons, 2017, CC BY-SA 4.0.
• Wikipedia, “Coefficient of relationship” and “Cousin”, CC BY-SA 4.0; Wiktionary, “grandaunt”.
• Cambridge Dictionary, “pibling” and “nibling”; Dictionary.com, “nibling”.
• ISOGG Wiki, “Autosomal DNA statistics” (3,400 cM per parent, 6,800 cM in all).
• R. L. Bennett and others, “Standardized Human Pedigree Nomenclature,” Journal of Genetic Counseling 17: 424–433, 2008, the symbols the tree uses.

Every source retrieved on 8 October 2026.