Interpolation
An editor's word or phrase inserted inside a quotation, marked with square brackets.
An interpolation is text that the quoting writer inserts inside quoted material to clarify, correct, or supply context that the original lacked. In modern English usage, interpolations are always enclosed in square brackets, precisely to distinguish them from words the original speaker or writer actually used. Parentheses would be ambiguous; square brackets say 'this is not the source'.
The most common interpolation is the identity clarification. If a source wrote 'He agreed to the terms', a quoting writer who needs the reader to know who 'he' refers to writes '[Wilson] agreed to the terms'. Other typical interpolations supply a translation ('the aim of philosophia [love of wisdom]'), a gloss ('the treaty [signed in 1919] failed'), or a grammatical adjustment when the quotation is being folded into a new sentence.
Interpolation carries an ethical rule: the meaning of the source must not change. Inserting '[obviously]' or '[incorrectly]' or any evaluative word is editorial commentary disguised as clarification, and careful readers spot it. The purpose of the brackets is to let the writer help the reader without seeming to speak in the source's voice.
Common mistake: over-interpolating. If a quotation needs four bracketed clarifications to make sense, the quotation is the wrong one — paraphrase would serve the reader better, with the original supplied in a note. Brackets are a scalpel, not a rewrite tool.
“"[Lincoln] believed that the Union could not endure permanently half slave and half free."”
Keep reading the glossary
Square brackets mark editorial changes inside a quotation — clarifications, edits, or [sic].
Latin for 'thus' — used in quotes to mark an error reproduced from the original.
A three-dot mark enclosed in square brackets — […] — that flags a deliberate editorial omission from a quotation.
A parenthetical note telling the reader that italics or bolding in a quotation come from the quoter, not the original author.