The official prefix bo- marks kinship created through marriage rather than direct descent or shared parentage. The most established forms are bopatro/bopatrino “father-/mother-in-law,” bofilo/bofilino “son-/daughter-in-law,” and bofrato/bofratino “brother-/sister-in-law.” The prefix tells you the type of relationship; the kinship root remains the semantic head. Mixed-sex in-law groups combine this prefix with the system described on ge-.
The core family words
Start from the ordinary kinship term and add bo-:
| Base | With bo- | Relationship |
|---|---|---|
| patro / patrino | bopatro / bopatrino | parent of one's spouse |
| filo / filino | bofilo / bofilino | spouse of one's child |
| frato / fratino | bofrato / bofratino | sibling-in-law through several possible marriage links |
Mia bopatrino instruis al mi sian recepton por la supo.
My mother-in-law taught me her soup recipe.
Ilia bofilo venos ripari la ĝardenan pordon sabate.
Their son-in-law will come to repair the garden gate on Saturday.
Mi renkontis mian bofraton ĉe la stacidomo.
I met my brother-in-law at the station.
The final endings work normally. Bofratojn is plural accusative, bopatrina means “related to/of a mother-in-law,” and boparenco is a gender-neutral general noun for an in-law relation.
Ni invitis ĉiujn boparencojn al la datrevena festo.
We invited all the relatives by marriage to the anniversary party.
— Ĉu via bofrato venos al la vespermanĝo? — Ne, sed mia bopatrino venos.
— Will your brother-in-law come to dinner? — No, but my mother-in-law will.
Why bofrato can be ambiguous
English “brother-in-law” is also ambiguous, and Esperanto bofrato conventionally covers several routes. It can denote the brother of one's spouse, the husband of one's sibling, or even the husband of a sibling of one's spouse. Bofratino has parallel readings.
Mia bofrato estas la pli juna frato de mia edzino.
My brother-in-law is my wife's younger brother.
Mia alia bofrato estas la edzo de mia fratino.
My other brother-in-law is my sister's husband.
The compound efficiently states the broad family category, not the exact path through the family tree. When that path matters, unpack it as la frato de mia edzino or la edzo de mia fratino. This is not a defect in Esperanto morphology; it reflects conventional kinship vocabulary.
The same practical simplification applies when two marriages create the relation. A logically imaginable bobofrato is not the normal everyday solution. Standard usage still uses simple bofrato or an explicit phrase.
La edzo de la fratino de mia edzo laboras en la sama hospitalo.
The husband of my husband's sister works at the same hospital.
That phrase is longer, but it communicates a relation that a stack of bo- prefixes would hide.
Bo- is not “step-” or “half-”
English groups several non-blood relationships under compounds, but Esperanto keeps them separate. Bo- requires marriage as the link. A step-relative created by a parent's new marriage normally uses vic-; a half-sibling sharing one parent is duonfrato/duonfratino.
Mia vicfrato estas la filo de mia vicpatro el lia antaŭa geedzeco.
My stepbrother is my stepfather's son from his previous marriage.
Mia duonfratino kaj mi havas la saman patron sed malsamajn patrinojn.
My half-sister and I have the same father but different mothers.
Neither person is automatically a bofrato or bofratino. Specifically, the child of a stepparent from an earlier relationship is vicfrato/vicfratino, not a sibling-in-law.
Less common but possible formations
The system can extend beyond the core three roots:
| Form | Possible meaning | Usage note |
|---|---|---|
| bokuzo / bokuzino | cousin by marriage | possible, less common |
| boonklo / boonklino | uncle/aunt by marriage | possible, less common |
| boavo / boavino | grandparent of one's spouse | possible, less common |
| bonevo / bonevino | nephew/niece by marriage | possible, less common |
Dum la geedziĝo mi unuafoje renkontis plurajn bokuzojn.
At the wedding, I met several cousins by marriage for the first time.
These formations are structurally valid, but speakers often use the simple kinship word loosely or explain the exact relation. That is an important limit on productivity: a logical derivative can be understood without being the community's default label.
With people outside a recognized kinship chain, bo- is not a general word for “connected to my spouse.” The spouse's colleague is kolego de mia edzo/edzino, not conventionally bokolego. The spouse's friend is amiko de mia edzo/edzino, not automatically boamiko.
Combining bo-, ge-, and pra-
Bo- can combine with other kinship prefixes. Bogepatroj means one's parents-in-law as a pair or group:
Miaj bogepatroj loĝas proksime al la maro.
My parents-in-law live near the sea.
The usual analysis is bo-[gepatroj]: marriage-related parents. Bo- scopes over the mixed parent group. Forms with pra- are possible for more distant generations, such as boprageavoj, but they become hard to decode without context.
En la familia arbo aperas ankaŭ la prageavoj de mia edzino.
My wife's great-grandparents also appear in the family tree.
The analytic phrase is often better than a dense bo-pra-ge- stack. Esperanto permits productive combination, but communicative usefulness—not theoretical possibility—should decide.
The relative order of bo- and pra- may not alter the intended family path in many cases, while ge- conventionally appears nearest the kinship root in forms such as bogepatroj. Preserve familiar forms rather than copying English compound order.
Root category and word families
Bo- primarily selects kinship nouns. Once the noun exists, normal endings produce related forms:
Ni organizas komunan tagmanĝon kun la bofamilio.
We are organizing a shared lunch with the in-law family.
La bopatrina konsilo montriĝis tre utila.
The mother-in-law's advice proved very useful.
Li havas bonan rilaton kun siaj bogefratoj.
He has a good relationship with his brothers- and sisters-in-law.
The adjective bopatrina is built from bopatrino and refers specifically to the mother-in-law. A broader adjective such as bofamilia concerns the family by marriage. The grammatical ending is the semantic head for word class; the prefix continues to specify the family link.
Do not expect bo- to turn an arbitrary verb or adjective into a marriage relation. It is productive within a semantic field, not unlimited across the lexicon.
Marriage ended or person deceased
Kinship vocabulary can persist socially after divorce or death, but bo- itself does not encode whether the connecting marriage is current. Context and community practice decide whether someone still calls a person bopatro. When legal or chronological precision matters, state it:
La patro de mia eksedzino ankoraŭ invitas min al la familiaj festoj.
My ex-wife's father still invites me to family celebrations.
Eksbopatro is structurally possible for “former father-in-law,” but an explicit phrase often avoids uncertainty about which relation ended. In (formal) legal writing, always identify the people and relationship directly.
A rare lexicalized extension
Historical and biblical texts contain boedziĝi for a specific custom of marrying a deceased brother's widow in order to continue his family line. This is (archaic/specialized) vocabulary, not a productive model for modern marriage verbs. Ordinary “marry” remains edziĝi/geedziĝi.
Recognizing this rare form is useful for reading, but learners should not infer that bo- can freely mean “marry into” before any verb.
Common Mistakes
Using bo- for a step-relative
❌ La filo de mia vicpatro estas mia bofrato.
Incorrect for a son from the stepparent's earlier relationship.
✅ La filo de mia vicpatro estas mia vicfrato.
My stepfather's son is my stepbrother.
Using bo- for a half-sibling
❌ Ni havas la saman patrinon, do li estas mia bofrato.
Incorrect — a shared parent creates a half-sibling relation, not an in-law relation.
✅ Ni havas la saman patrinon, do li estas mia duonfrato.
We have the same mother, so he is my half-brother.
Expecting bofrato to name the exact path
❌ Bofrato ĉiam estas nur la frato de mia edzino.
Incorrect — the conventional word covers several marriage-created sibling relations.
✅ Mia bofrato estas la edzo de mia fratino.
My brother-in-law is my sister's husband.
Copying English compound order
❌ La gepatroboj vizitos nin dimanĉe.
Incorrect order — bo- modifies the already formed gepatroj.
✅ La bogepatroj vizitos nin dimanĉe.
The parents-in-law will visit us on Sunday.
Extending bo- to any associate of a spouse
❌ Mi trinkis kafon kun mia bokolego.
Unconventional if this merely means my spouse's colleague.
✅ Mi trinkis kafon kun kolego de mia edzino.
I had coffee with a colleague of my wife.
Key Takeaways
Use bo- for a kinship relation created by marriage. Learn the core conventional forms, accept that bofrato/bofratino can name several family paths, and unpack the relation when precision matters. Keep vic- for step-relations and duon- for shared-one-parent relations. Productive extensions and prefix stacks are possible, but familiar compounds or explicit phrases usually communicate better.
Related Topics
- ge-: Mixed-Sex GroupingA2 — Use traditional ge- for pairs and groups containing more than one sex, while recognizing modern gender-neutral extensions and their limits.
- pra-: Remoteness In Ancestry Or TimeA2 — Use pra- for remote time, origins, and generations beyond ordinary grandparent or grandchild terms.
- -in-: Female SexA2 — Use -in- to specify female sex while preserving neutral person and animal roots when sex is irrelevant.
- Affix Stacking and ScopeB2 — Interpret and build multi-affix Esperanto words by applying each layer in order and keeping the semantic head and participant roles clear.
- People and Animate ReferenceA2 — How Esperanto names people and animals without grammatical gender while marking number, case, and relevant natural sex.
- Natural Sex and Lexical GenderB1 — How sex can be part of a root’s meaning or added explicitly without creating grammatical gender agreement.