Notation guide for kin types, relations and diagrams

Last update: Oct. 14, 2026

Kinship is easy to talk about and hard to talk about precisely. "My cousin" can mean any of eight first-cousin positions and many more distant ones; "my uncle" can be a blood relative or someone who married in. The notation below removes that ambiguity. It is the shorthand used across this site, and where the literature uses other conventions, the variants are listed so you can read older and newer work without tripping.

Kin-type letters

Every genealogical position is written as a string of single capital letters, one per link.

Letter	Stands for	Sex-neutral counterpart
F	father	P, parent
M	mother	P, parent
B	brother	G, sibling
Z	sister	G, sibling
S	son	C, child
D	daughter	C, child
H	husband	E, spouse
W	wife	E, spouse

Sister is Z because S is already taken by son. The sex-neutral letters P, C, G and E follow the recommendations in Barnard and Good's handbook of kinship research methods (1984); they are handy when a rule applies to both sexes, as in "PG", any parent's sibling. Some reference works use two-letter codes instead, such as SI for sibling and SP for spouse.

Reading a string

A string is read from Ego outward, left to right, as a chain of possessives. Ego is implicit and never written unless a formula needs it.

String	Read as	Usual American English term
FB	father's brother	uncle
MBD	mother's brother's daughter	cousin
FZS	father's sister's son	cousin
WB	wife's brother	brother-in-law
HZ	husband's sister	sister-in-law
ZDH	sister's daughter's husband	niece's husband
FFBSS	father's father's brother's son's son	second cousin

There are no spaces and no apostrophes inside a string. When a string is long, read it in pairs from the left: FF, then B, then S, then S.

Elder and younger: e and y

Many terminologies separate older from younger siblings, so the notation needs a way to mark relative age. A lowercase e (elder) or y (younger) goes immediately before the letter it qualifies: eB is an elder brother, yZ a younger sister.

On this site the marker compares a person with the one just before them in the string. FeB is a brother older than Ego's father, not necessarily older than Ego. This matters when you reverse a relationship, as the worked example at the end shows.

Sex of the speaker: ms and ws

The same position can take different terms depending on whether a man or a woman is speaking. We add (ms) for "man speaking" or (ws) for "woman speaking" after the string: ZS (ms) is a man's sister's son. You will also meet (m.s.) and (f.s.), for male speaking and female speaking; they mean the same thing.

Where a terminology turns on whether two siblings are of the same or opposite sex, we write ss or os before the sibling letter: an osG is an opposite-sex sibling, so for a man, his sister.

Generation and collateral distance

To place any consanguineal relative, count two numbers. Let u be the number of steps up from Ego to the nearest ancestor Ego shares with the relative (for a direct ancestor, the relative is that ancestor), and d the number of steps down from that ancestor to the relative. Then:

generation = u - d, written G+1, G0, G-1 and so on (positive means senior to Ego; some texts write just +1, 0, -1)
collateral distance = the smaller of u and d (0 means lineal, a direct ancestor or descendant)
for cousins, degree = the smaller of u and d, minus 1; removal = the difference between u and d

Kin type	u	d	Generation	Collateral distance	Genealogist's name
F	1	0	G+1	0 (lineal)	parent
FF	2	0	G+2	0 (lineal)	grandparent
S	0	1	G-1	0 (lineal)	child
B	1	1	G0	1	sibling
FB	2	1	G+1	1	uncle
BS	1	2	G-1	1	nephew
FBS	2	2	G0	2	first cousin
FBSS	2	3	G-1	2	first cousin once removed
FFBS	3	2	G+1	2	first cousin once removed
FFBSS	3	3	G0	3	second cousin

The same two numbers drive the cousin calculator. They also give the coefficient of relationship for relatives linked through a pair of full siblings: r = 2 × (1/2)^(u + d), so a first cousin is 2 × (1/2)^4 = 1/8. For affines, count through the linking spouse: WB stands in Ego's generation because W does.

Diagram symbols

Symbol	Meaning	Variants you may meet
△	male
○	female
□ or ◇	sex unknown or not relevant	clinical genetic pedigrees use a square for male and a diamond for unknown sex
= between two symbols	marriage or union	a line joining the two from below
≠	divorce or separation	a marriage line crossed by one or two short strokes
vertical line	descent, from a couple down to their children
horizontal bar above several symbols	siblings, each hanging from the bar
diagonal slash through a symbol	deceased	the symbol filled in or shaded
Ego	the person the diagram is drawn from	labeled "Ego", marked with an arrow, or shaded

Because shading can mean either "deceased" or "Ego" depending on the author, every diagram on this site carries a key. Generations are drawn as horizontal rows with the oldest at the top. For a step-by-step drawing exercise, see how to draw a kinship diagram.

Relations and how they compose

Each letter can be read as a relation: M(x) is the mother of x, B(x) the set of brothers of x. Linking two relations is composition, written with ∘ and read right to left, the way mathematicians compose functions. So (F∘M)(x) = F(M(x)), "the father of the mother of x". In a kin-type string that same person is MF. The two notations run in opposite directions, which is the commonest source of slips.

String	Composition	In words
MF	F∘M	mother's father
FM	M∘F	father's mother
MBD	D∘B∘M	mother's brother's daughter
WB	B∘W	wife's brother

F∘M and M∘F are different people, so composition of kin relations is not commutative in general. Whether it becomes commutative for the classes of a particular marriage system is exactly the question behind the note on Abelian kinship systems.

The same notation describes marriage-class systems. In Philippe Courrège's formulation (1965), a permutation ω says that men of class x marry women of class ω(x), and a permutation μ says that women of class x have children in class μ(x). A man's children then belong to class μ(ω(x)), written π = μ∘ω: first find his wife's class, then her children's. The explainer on André Weil's algebra works through a four-class example.

Two more conventions are worth fixing early.

Converses. P and C are converses of each other; G and E are their own converses. To reverse a whole string, write it backward, replace each letter by its converse, and take each person's sex from the person you arrive at. MB reversed: from the uncle, step to his sister (Ego's mother), Z; then to her child, Ego, S or D. So Ego is MB's ZS or ZD, always man speaking, because MB is a man.
Assumptions. Identities such as FW = M or MH = F hold only in a model without remarriage; once step relations are allowed, FW may be a stepmother. State the assumption before you simplify a string.

Terms and types

A kin type is a genealogical position (MB). A kin term is a word in a particular language (uncle). On this site types are always in capitals and terms in italics; a term from another language is italicized and glossed in quotation marks. The set of types a term covers is its range: English uncle covers FB, MB, FZH and MZH, while cousin covers all eight first-cousin types and many more distant ones.

To show that two types share a term in some terminology, we write a tilde: in an Iroquois-type terminology FB ~ F and FBS ~ B, while in English FB ~ MB. Reduction-rule analyses in the tradition of Lounsbury (1964) write rules with arrows that rewrite a longer type to a shorter one with the same term, and separate a term's focal referent from its extended ones. The six kinship systems explainer and the kinship terms chart show these mergers across whole terminologies, and cross and parallel cousins covers the distinction most of them turn on.

Kroeber (1909) argued that terminologies differ in which of a small set of distinctions they use. Later writers name his eight as follows.

Principle	Question it asks	English
Generation	same, senior or junior generation?	used: father vs brother
Affinity	related by blood or by marriage?	used: brother vs brother-in-law
Collaterality	direct line or side line?	used: father vs uncle
Sex of relative	male or female?	used: uncle vs aunt
Bifurcation	sex of the linking relative?	not used: uncle for FB and MB
Sex of speaker	is a man or a woman speaking?	not used
Relative age	older or younger?	not used for siblings
Decedence	is the linking relative alive?	not used

Later writers add a ninth, polarity: whether the two people in a relationship use different terms for each other (aunt and niece) or the same one (cousin and cousin).

A worked example

Take FyBD (ws), a woman's father's younger brother's daughter.

Read it: Ego is a woman; her father's younger brother has a daughter.
Count: up to the shared grandparents is u = 2, down to the relative is d = 2. Generation G0, collateral distance 2, a first cousin, not removed.
Classify: the linking siblings, F and FB, are both male, so she is a parallel cousin. American English says cousin; an Iroquois-type terminology would use the sister term.
Reverse: from the cousin, step to her father (F), then to his brother, Ego's father (B), then to Ego (D). The converse is FBD again, but the age marker flips, because Ego's father is the elder of the two brothers. From the cousin's side the relationship is FeBD (ws).