Definition
Interpolation has two distinct legal meanings that rarely intersect but both appear in legal research contexts.
1. Textual interpolation. The act of inserting words, passages, or marks into a document, manuscript, or record that were not part of the original. In law, this most often arises as an allegation of fraud or forgery—that a party has added language to a written instrument after its execution, without the knowledge or consent of the other signatories. The inserted material itself is also called an interpolation. The term is neutral in form but typically carries a negative implication in legal usage: it suggests unauthorized or deceptive alteration rather than a deliberate, disclosed amendment.
2. Civil law: notice of termination. In the civil law tradition, interpolation refers to a formal declaration by one party to a continuing obligation—particularly a periodic lease or contract of indefinite duration—that the party will not be bound beyond a specified time. It functions as a notice of termination or a demand served on the counterparty to put an end to the legal relationship. Bouvier's Dictionary identifies this as the proper civil law meaning, analogous to what common law jurisdictions call notice to quit.
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Common Language
Modern common usage (Wiktionary): Interpolation most commonly refers to estimating an unknown value between two known data points (mathematical interpolation), or to inserting externally fetched data into a program or document (computing). The general sense of introducing something into a sequence or text also persists in literary and editorial contexts.
Historical common usage (Webster's 1913): "The act of introducing or inserting anything, especially that which is spurious or foreign." Webster's examples draw on textual criticism—insertions into ancient manuscripts attributed to later hands.
The gap matters for researchers: in common and scholarly usage, interpolation is descriptively neutral and includes both innocent editorial additions and corrupt insertions. In legal usage, the word almost always signals a contested or unauthorized alteration—an act with potential consequences in fraud, forgery, or evidence doctrine. The civil law meaning (termination notice) has no common-language counterpart at all and is easily missed by researchers encountering the term in civilian or Louisiana sources.
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Common Confusion
Interpolation is sometimes conflated with interlineation, which refers specifically to writing between existing lines of a document. Interpolation is the broader concept—it encompasses any insertion, whether interlinear or appended—while interlineation describes the physical placement of added text. Courts and treatises have occasionally used the terms interchangeably, so researchers should not assume precision when either word appears in older sources. Interpolation should also be distinguished from erasure and alteration: an erasure removes original text; an alteration changes original text; an interpolation adds new text to an otherwise intact original.
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Why It Matters in Research
Researchers encounter interpolation most frequently in two distinct documentary contexts, and conflating them causes navigational errors.
In document authentication and evidence research, interpolation appears in cases involving contested wills, deeds, contracts, and commercial paper. Historical treatises on the law of evidence discuss interpolation as a form of fraud on an instrument. The question was not merely whether text had been added, but when—before or after execution—and whether the alteration voided the instrument entirely or only the added portion. Rules varied by document type and jurisdiction, so results from general evidence treatises may not transfer cleanly.
In civil law and Louisiana research, the termination-notice meaning of interpolation is the operative one. Researchers working in Louisiana property law, lease doctrine, or obligations should treat Bouvier's civil law definition as controlling context. This meaning is effectively invisible in common law dictionaries and is underrepresented even in specialized legal reference works, making it a trap for researchers trained in common law sources who assume the term always signals document fraud.
The mathematical and computing senses of interpolation appear with increasing frequency in modern regulatory and commercial law contexts—for example, in rate-setting, financial modeling disclosures, and data processing agreements. These uses carry no legal term-of-art significance; they import the technical sense directly.
Black's entries in both editions are unusually sparse—merely defining interpolation as "the act of interpolating; the words interpolated"—without distinguishing the civil law meaning or situating the term in evidentiary doctrine. Researchers relying solely on Black's will miss the civil law usage entirely.
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Historical Dictionary Support
Rapalje & Lawrence and both editions of Black's converge on a single definition: the act of interpolating, and the words so interpolated. This is the textual-alteration sense. None of these sources develop the definition or connect it to evidentiary rules, fraud doctrine, or specific instrument types—a notable gap given how extensively courts litigated interpolation questions in will contests and commercial paper disputes through the nineteenth century.
Bouvier stands apart by preserving the civil law meaning explicitly, drawing on Wolff's Institutiones Juris Naturae et Gentium. This is the only historical dictionary source in the Law Mind corpus that captures the termination-notice usage, and it should be treated as authoritative for civilian contexts. Bouvier's example—a lease from year to year, or one continuing at the parties' pleasure—anchors the concept usefully.
Webster's 1913 is actually more informative than the strictly legal dictionaries on the textual-insertion sense, particularly in its implication that interpolations are characteristically "spurious or foreign." That framing aligns closely with how courts treated contested insertions: the presumption tended to run against the party seeking to enforce interpolated language.
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Jurisdictional Note
The civil law termination-notice meaning is specific to civilian jurisdictions, most significantly Louisiana. Common law courts do not use "interpolation" in this sense and would treat it as a term of art requiring translation. In common law jurisdictions, the term is confined to evidence and document law.
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