Definition
In old English law, "aisne" (also spelled "eigne") means the eldest or firstborn child. The term appears in historical legal texts as a designation of birth order carrying legal consequence, particularly in the context of inheritance and land succession. The eldest son's status as aisne was legally significant because primogeniture — the rule by which the firstborn male heir inherited the entirety of a deceased ancestor's real property — was the default rule of succession in English common law for centuries.
The term is essentially obsolete in modern legal usage, surviving only in historical sources and as a curiosity of Norman-influenced legal vocabulary.
Common Language
Modern common usage (Wiktionary): "Aisne" refers to a department in the Hauts-de-France region of northeastern France, and to a river — a left tributary of the Oise — flowing through that region.
Historical common usage: The geographic and river senses of "Aisne" have no meaningful overlap with its legal meaning. The legal term and the French place name are homographs, not related concepts.
Editorial note: A researcher encountering "aisne" in a French-language source or modern geographic context should not import any legal meaning. Conversely, a researcher working in old English legal records will find no geographic reference — the legal term is entirely separate in origin and application from the modern common usage.
Common Confusion
AISNE / EIGNE: These are variant spellings of the same term, not distinct legal concepts. Historical sources are inconsistent in their spelling, reflecting the Norman French origins of the word and the variable orthography of medieval English legal writing. Researchers should treat both forms as equivalent and search for both when working in manuscript or early printed sources.
Why It Matters in Research
The practical significance of aisne is almost entirely historical. Researchers working in medieval English land records, Year Books, or early common law treatises will encounter the term in discussions of primogeniture, inheritance disputes, and questions of legitimacy. The firstborn status of a claimant could be dispositive in actions involving descent of freehold estates.
The spelling variation between "aisne" and "eigne" is a genuine research trap. Because historical legal texts were not standardized, the same term may appear in multiple forms across different sources, and indexing in older digests and abridgments may be inconsistent. Searching only one spelling risks missing relevant material.
Researchers should also note that "bastard eigne" and "mulier puisne" are a paired set of concepts closely related to this term. Where a man had a child by a woman before marriage and a legitimate child after, the elder illegitimate child was the "bastard eigne" and the younger legitimate child was the "mulier puisne." The legal rules governing which could inherit under these circumstances occupied significant attention in the old common law and generated a distinct body of doctrine. Aisne, standing alone, simply means firstborn — but it gains much of its legal texture from that bastard eigne / mulier puisne context.
The term has no meaningful modern application. Primogeniture as a rule of intestate succession for real property was abolished in England by statute in the nineteenth century and was never universally adopted in American jurisdictions. Any modern research need connecting to birth-order rights in succession should consult contemporary statutes and the entry for PRIMOGENITURE, not this term.
Historical Dictionary Support
Black's Law Dictionary records the term concisely as "the eldest or first born," noting both the "aisne" and "eigne" spellings and situating it in old English law. The entry is minimal, reflecting that by the time Black's was compiled, the term had already receded from active legal use into the category of historical vocabulary.
What Black's does not supply — and what researchers should seek elsewhere — is the substantive doctrinal context: the rules of primogeniture, the bastard eigne doctrine, and the broader system of real property descent within which this term did its legal work. For that, the major treatises of English legal history, including Bracton, Littleton, and Blackstone's Commentaries, provide the necessary framework.