Definition
Filius mulieratus is a term from old English law denoting the eldest legitimate son of a woman who previously had an illegitimate son by the same father. The term is sometimes rendered simply as "mulier." The scenario it describes is specific: a man fathers a child outside of marriage, then later marries the mother and fathers a second child with her. The first child, born before the marriage, is the bastard eigne (or bastardus eigne); the second child, born in lawful wedlock, is the filius mulieratus — literally, the "son of the lawful wife." This distinction carried significant consequences for inheritance, particularly in the context of seisin and the common law rules governing land succession.
Common Confusion
FILIUS MULIERATUS vs. BASTARDUS EIGNE: These two terms describe opposite sides of the same family situation and must be read together. The bastardus eigne is the elder, illegitimate son; the filius mulieratus is the younger, legitimate son. The confusion arises because the illegitimate elder son sometimes had practical advantages at common law: if the bastardus eigne obtained seisin of land before the father's death and held it without challenge, his younger legitimate brother — the filius mulieratus — could not disturb that possession after the father died. The legitimacy of the filius mulieratus did not automatically displace the elder brother's prior seisin. Researchers conflating the two terms will misread the legal outcome entirely.
Why It Matters in Research
This term belongs to a narrow but analytically important corner of medieval and early modern English inheritance law. Researchers working in that period should understand three things.
First, the term is historical without modern application. Filius mulieratus has no operative role in contemporary law. It appears almost exclusively in treatises, year books, and case reports from the medieval period through the early common law commentaries. When it surfaces in nineteenth-century American legal dictionaries, it is being preserved as a reference tool for reading older English sources, not as living doctrine.
Second, the interaction with bastardus eigne is the research point that matters. The doctrine those two terms together describe — sometimes called the "bastard eigne and mulier puisne" rule — turned on seisin rather than status. A researcher who reads filius mulieratus as simply meaning "the legitimate one who wins" will be wrong. The outcome depended on whether the bastard eigne had obtained and held undisturbed seisin. Glanvill (lib. 7, c. 1) and Blackstone (2 Bl. Comm. 248) are the foundational sources both historical dictionaries point to, and those are the right places to trace the doctrine.
Third, the term is a reliable signal of period and jurisdiction. Its appearance in a primary source marks that source as English and almost certainly pre-modern. American sources that use this term are either quoting English authority or synthesizing it for comparative purposes.
Historical Dictionary Support
Black's Law Dictionary and Burrill's Law Dictionary are in complete agreement on this term, and their entries are nearly identical — both citing Glanvill lib. 7, c. 1 and 2 Bl. Comm. 248, and both noting the synonym "mulier." The convergence is unsurprising: this is a term of sufficiently narrow historical usage that there was little room for interpretive divergence. Neither dictionary elaborates on the seisin rule or the practical interplay with bastardus eigne, which is the doctrinally interesting part. Researchers relying solely on these dictionary entries will get the identity of the person but not the legal consequence. Blackstone's own text and Glanvill, both of which the dictionaries cite, supply what the dictionary entries omit.
Jurisdictional Note
Filius mulieratus is exclusively a term of old English law. It was never adopted as operative doctrine in American jurisdictions, though American legal dictionaries preserved it as a reading aid for English common law sources. No meaningful jurisdictional variation exists because the term never crossed into active American legal usage.