Definition
To insert words, passages, or other matter into an existing, complete document. In legal usage, interpolation refers specifically to the unauthorized or undisclosed addition of material to a document after its execution or completion — an act that, depending on context and intent, may constitute fraud, forgery, or render a document void or voidable. The interpolated material may be a word, phrase, clause, or entire passage introduced to alter meaning, create an obligation, or misrepresent the original instrument.
Interpolation is distinct from amendment or addendum in that it implies the insertion is concealed within existing text rather than visibly appended. When a signatory or third party inserts language after execution without the knowledge or consent of other parties, the legal consequences can be severe — particularly in the law of wills, deeds, contracts, and negotiable instruments.
Common Language
Modern common usage (Wiktionary): To introduce something between other things; especially to insert (possibly spurious) words into a text. Also used in mathematics to estimate a value between two known data points, and in computing to fetch and process external data inline.
Historical common usage (Webster's 1913): To alter or corrupt by the insertion of new or foreign matter; especially to change a book or text by the insertion of matter that is new or foreign to the purpose of the author. Also used (obsoletely) to mean to renew or carry on with intermission.
The legal meaning tracks the literary/textual sense closely but adds a layer of legal consequence absent from ordinary usage. In common speech, interpolation may describe innocent editorial insertion — a translator filling a gap, a scholar clarifying a passage. In law, interpolation almost always carries an implication of impropriety: the insertion is unauthorized, undisclosed, and typically intended to change the legal effect of an instrument. The mathematical and computing senses are wholly irrelevant to legal research.
Common Confusion
INTERPOLATE vs. INTERLINEATION: These terms are sometimes used interchangeably but are not identical. Interlineation refers to writing between the lines of an existing document — a specific physical form of insertion. Interpolation is the broader concept encompassing any insertion into completed text, regardless of placement. All interlineations are a form of interpolation, but interpolation may also include insertions within existing lines or at other points in a document.
INTERPOLATE vs. ALTERATION: Alteration encompasses any change to a document — erasure, substitution, or addition. Interpolation is a subset of alteration, specifically limited to additions. An erasure followed by substitution is an alteration but not technically an interpolation.
Why It Matters in Research
The practical stakes of interpolation arise most often in three areas: (1) contested wills, where post-execution insertions may invalidate specific provisions or the entire instrument; (2) negotiable instruments and commercial paper, where unauthorized additions can constitute fraud and affect holder-in-due-course status; and (3) deed and contract disputes, where inserted terms change the scope of obligations or conveyances.
Researchers working with historical sources should be aware that the term appears with varying weight. In some older sources, interpolation describes what modern law would call a material alteration — a distinction that matters for the remedy available. Courts in the 19th and early 20th centuries often treated unauthorized interpolation as rendering a document void ab initio rather than merely voidable, a position that has softened in some modern jurisdictions where the innocent party's rights are more carefully preserved.
The term also appears in a distinct scholarly context — the criticism of historical legal texts, particularly Roman law — where interpolation refers to changes introduced by Justinian's compilers into classical texts. Researchers working in comparative law or legal history may encounter this usage in discussions of the Corpus Juris Civilis and must distinguish it from the transactional law sense.
In corpus research, check whether a source is using interpolation to describe an act (the insertion itself), a result (the altered document), or a scholarly inference (the identification of inserted passages in historical texts). These senses blend in older treatises and can create confusion when tracing doctrinal development.
Historical Dictionary Support
All three source dictionaries — Black's (1st Ed.), Black's (2nd Ed.), and Rapalje & Lawrence — offer the same compact definition: "to insert words in a complete document." The brevity is striking and consistent. None of the historical entries address the element of unauthorized intent, the legal consequences, or the distinction from related acts like interlineation or material alteration.
This silence is instructive. The historical dictionaries define the act mechanically without legal freight. The legal consequences of interpolation — fraud, forgery, voidness — were presumably addressed in treatises and case law rather than dictionary entries. Researchers relying solely on dictionary sources will find only the bare act defined; they must look elsewhere for the doctrinal framework governing when interpolation matters and what follows from it.
Webster's 1913 offers the richer description, capturing the sense of corruption and foreign insertion that the legal dictionaries leave implicit. The Webster's framing — "to alter or corrupt by the insertion of new or foreign matter" — is closer to the operative legal concept than any of the three legal dictionary entries.