Definition
In civil law, linea obliqua (Latin: "oblique line") is an alternate term for the collateral line of descent — the line connecting persons who share a common ancestor but are not descended one from the other. Siblings, cousins, aunts, uncles, and nephews are related through linea obliqua rather than through the direct ascending or descending line. The term is largely interchangeable with linea transversalis in the historical sources, and both refer to the same structural concept in civil-law kinship reckoning.
Common Confusion
Linea obliqua is consistently treated in historical legal dictionaries as a synonym for linea transversalis, with linea transversalis being the preferred and more commonly used form. Researchers encountering linea obliqua in a historical source should not treat it as a distinct concept with its own rules or nuances — it is simply an alternate Latin phrasing pointing to the same collateral-line doctrine. See LINEA TRANSVERSALIS for the substantive treatment. A third related term, linea recta, refers to the direct line (ancestors and descendants); confusion between linea recta and linea obliqua/transversalis is possible in dense civil-law succession texts.
Why It Matters in Research
This term functions almost exclusively as a cross-reference term. Its practical value in corpus research is as a signal, not a destination: when linea obliqua appears in a historical document, it marks a discussion of collateral descent and inheritance under civil-law principles, and the researcher should follow the thread to linea transversalis for doctrine and analysis.
The term surfaces most often in treatises on civil-law succession, Roman-law inheritance, and — in American legal history — Louisiana sources, which imported civilian kinship reckoning directly into their intestate succession framework. Researchers working in pre-20th-century Louisiana succession materials, or in any common-law jurisdiction grappling with civil-law influence, may encounter linea obliqua in place of the more familiar common-law vocabulary of "collateral kindred" or "collateral heirs."
The term does not appear to have generated independent case law or statutory usage. Its appearances are predominantly in explanatory and definitional passages in treatises, commentaries, and legal dictionaries. A search that returns linea obliqua in a corpus is almost certainly returning foundational explanatory text rather than operative legal argument.
Historical Dictionary Support
Black's Law Dictionary, Rapalje & Lawrence, and Burrill's Law Dictionary are in complete agreement on this term: all three define it identically as "the oblique line" and all three immediately redirect the reader to linea transversalis as the more common usage. None of the three sources develops independent doctrine under linea obliqua or treats it as carrying meaning distinct from linea transversalis. This unanimity across three major historical dictionaries is itself informative — the term had no contested meaning, no jurisdictional variation worth flagging, and no independent doctrinal life. It existed in the lexicon as a philological variant, not as a term of art with its own analytical weight.
What the historical dictionaries do not address, and what a researcher should supply independently, is the broader civil-law framework within which both terms operate — specifically, the distinction between the linea recta (direct line) and the linea obliqua/transversalis (collateral line) as the foundational partition of kinship in civil-law succession systems.
Jurisdictional Note
The term belongs to the civil-law tradition and is most likely to appear in Louisiana legal sources or in American treatises discussing Roman and continental inheritance law. Common-law jurisdictions use "collateral line" or "collateral kindred" to express the same concept and will not typically employ linea obliqua.