Definition
Filius Petri is a Latin phrase meaning "son of Peter." In medieval English legal usage, it functioned as a personal identifier in pleadings, writs, and court records, denoting a named individual as the son of a man named Peter. The phrase is the Latin equivalent of the Anglo-Norman patronymic "Fitz Peter," both forms appearing interchangeably in Bracton-era legal documents to identify parties before the court.
The term carries no independent substantive legal meaning — it is a naming convention, not a legal concept. Its legal significance lies entirely in its function as a mode of personal identification at a time when fixed hereditary surnames had not yet standardized in England.
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Common Confusion
Filius Petri and Fitz Peter are the same identifier rendered in two languages. Medieval legal scribes writing in Latin would use Filius Petri; the same designation in Anglo-Norman French appears as Fitz Peter. Researchers encountering one form should search for both when tracing a party across documents of the same period. Neither form is a surname in the modern sense — the designation changed with each generation (the son of Filius Petri would be Filius [name of the father], not Filius Petri).
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Why It Matters in Research
The primary research challenge with filius-pattern identifiers is that they are generational and non-hereditary. A man identified as Filius Petri in one document may appear in another document simply by a given name, or by a territorial identifier, or not at all under any recognizable variant. Cross-referencing requires attention to the father's name, approximate date range, and geographic context rather than relying on the name string alone.
Researchers working in the Law Mind corpus with Bracton-era materials should note that Bracton's text (cited by Burrill at folios 225b, 420b, 298, 418, and 421) uses filius-form identifiers extensively when citing cases and examples. These references are illustrative rather than nominative — Bracton names parties not to build a case reporter in the modern sense but to anchor legal propositions in recognizable examples. Do not treat these party names as reliable pointers to external records without corroboration from chancery rolls or plea rolls of the period.
Jurisdictionally, this naming convention is specific to medieval English and Anglo-Norman legal records. Continental civil law materials use similar Latin patronymic forms, but the specific corpus context here is common law pleading during the period when Bracton's De Legibus et Consuetudinibus Angliae was authoritative, roughly the mid-thirteenth century. By the late medieval period, hereditary surnames had sufficiently stabilized in English practice that filius-form identifiers in legal documents became rare and their appearance may itself signal an unusually early source or an archaizing scribe.
The pairing of Filius Petri and Filius Radulphi (Fitz-Ralph) in Burrill's entry is notable: Burrill treats these together as a class of identifier, not as entries for specific historical persons. This is the correct interpretive frame. Researchers should not search for a single individual bearing "Filius Petri" as a fixed name but should understand the form as a category of identification that could apply to any number of persons across documents.
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Historical Dictionary Support
Burrill's Law Dictionary provides only a brief cross-reference entry, glossing Filius Petri as "Fitz Peter" and citing Bracton folios 225b and 420b, immediately followed by the parallel entry for Filius Radulphi (Fitz-Ralph) with citations to folios 298, 418, and 421. The entry is definitional in the minimal sense: it confirms the Latin-to-Anglo-Norman equivalence and points readers to Bracton for context. Burrill offers no analysis of the naming convention itself, its legal function, or its period of use.
No other source dictionaries were provided for this term. The absence of entries in later dictionaries (such as Black's) is itself informative: by the time American legal lexicography matured in the nineteenth century, filius-form identifiers had no practical relevance to current legal practice and were treated, if at all, as historical curiosities or translation aids for scholars working with medieval sources.
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