Plural of “addendum”

The plural of addendum is addenda (some dictionaries also accept addendums).

Why

"Addendum" is Latin for "a thing that must be added," a gerundive form of the verb "addere" (to add) — grammatically it originally named an obligation to add something rather than simply labeling the added item itself. Its plural, "addenda," follows straightforwardly from the Latin rule that neuter nouns ending in -um switch to -a in the plural, exactly as with "datum/data" and "medium/media," and English absorbed that inherited ending intact when "addendum" entered formal legal and publishing usage in the 17th century.

"Addenda" remains the dominant plural specifically because the word is still mostly used in formal writing — contracts, academic papers, and technical manuals — registers that have historically resisted regularizing Latin plurals faster than everyday speech does. "Addendums" does appear, particularly in casual business writing, but style guides for legal and academic publishing still generally prefer "addenda," making this one of the more conservative members of the Latin -um family.

Usage note

Use "addenda" when referring to the collection of items added to a document as a set ("the addenda to the contract"), which is how the word appears in most professional writing. "Addendums" is acceptable in more informal or conversational business contexts but will read as slightly nonstandard in a legal or academic setting.

Academic publishing distinguishes an addendum from an "erratum," a separate published correction of an actual mistake, versus an addendum, which adds genuinely new material the author simply hadn't included the first time around — the two terms describe different kinds of after-the-fact editorial additions and shouldn't be used interchangeably in formal scholarly writing.

Common real mistakes

A frequent mistake is treating "addenda" itself as singular ("see the addenda on page nine" referring to one single item), which blurs the distinction the word is meant to preserve; the singular for one added item is always "addendum." Another slip is inventing "addendas" by adding an English plural ending onto the already-plural Latin form — a double-marking error that shows up occasionally in rushed business documents. A further point of care: an addendum should be dated and clearly marked as supplementary, since an undated addendum can create genuine ambiguity about which version of a document controls.

The wider pattern

"Addendum/addenda" sits alongside "datum/data," "medium/media," "stratum/strata," and "curriculum/curricula" as part of the Latin second-declension neuter family that took -a plurals in the source language; each word shows a different degree of resistance to regularizing toward an English -s plural depending on how tightly it stays bound to formal, technical writing versus everyday speech. Publishing and legal drafting distinguish an "addendum" from an "amendment": an addendum adds new material without altering what came before, while an amendment actually changes existing terms — a distinction worth knowing since the two words are sometimes used loosely as if interchangeable, even though a contract's addenda and its amendments can have very different legal effects.

Examples

• The contract included three addenda covering payment terms.
• Please review the addendum before signing the final agreement.
• Several addenda were attached to the report after the initial review.
• The publisher issued an addendum correcting the printing error.
• All addenda must be initialed by both parties.
• A single addendum clarified the delivery schedule.
• The lease came with two addendums about pet policies.
• Committee members were asked to submit their addenda by Friday.
• The revised manual includes an addendum on safety procedures.
• Legal teams often negotiate addenda separately from the main contract.
• The editor inserted a short addendum explaining the author's later change of mind.
• Two competing addenda were submitted before the board could vote.
• The building permit required an addendum detailing the updated fire code compliance.
• The journal published an addendum clarifying the study's original sample size.
• Reviewers requested an addendum addressing a gap in the methodology section.
• The final printing included an addendum listing every known typo from the first run.