Glossary

Common knowledge: the facts your readers will accept without a source

Some facts your readers will accept without a source: undisputed dates, standard definitions, and findings that appear unattributed throughout the literature of your field. That is common knowledge, and it needs no citation. The boundary moves with audience — what passes in a specialist journal may still need a source in an undergraduate essay.

Why it matters when you are writing

The workable test has two parts. Can the fact be found, uncited, in several independent reference works or textbooks in the field? And would your intended reader accept it without wanting to check? Both yes, and you can leave it unattributed. Either no, and cite. This is a judgment, and judgments are supposed to be made deliberately rather than by default.

Audience does most of the work. That the Berlin Wall fell in 1989 needs no source anywhere. That a particular protein folds through a specific intermediate is common knowledge in a structural biology lab and needs a citation in a general science essay. Writing for an interdisciplinary journal usually means citing more than you would in your home subfield, because your reader’s baseline is different.

One category is not covered by the exception no matter how familiar it feels: specific numbers. A prevalence rate, an effect size, a survey percentage, a measured constant — these come from a study, and the study gets cited even when the figure circulates widely. If your sentence contains a digit that somebody had to measure, it needs a source.

IN PRACTICE

Two facts, one citation

No citation needed: water boils at 100 °C at sea level. Citation needed: boiling point elevation across the sampled brines averaged 3.4 °C (Hardin, 2019) — a specific measured figure, however often it is repeated.

OFTEN CONFUSED WITH

What Common knowledge is not

General knowledge: General knowledge is what an ordinary reader knows; common knowledge in academic writing is defined by your discipline and your audience.
Public domain: Public domain is a copyright status about reuse rights; common knowledge is about whether a fact needs attribution at all.

Questions about common knowledge

What is the safe default when I am unsure?

Cite. An unnecessary citation costs a line in the reference list; a missing one is an integrity question you will have to answer.

Do statistics ever count as common knowledge?

Effectively never. Specific figures come from a study or a dataset, and that source gets named however widely the number is quoted.

Does common knowledge differ between fields?

Substantially. A concept assumed without citation in one discipline may be a contested claim requiring support in the discipline next door.

Related terms

In-text citation

Every borrowed idea in a paper carries an in-text citation: a brief marker — usually an author name plus a year, page, or number — that points to the full entry in your reference list.

Plagiarism

Plagiarism is presenting someone else’s words, ideas, structure, data, or images as your own — by omitting attribution, by attributing so loosely that the borrowing is hidden, or by reproducing phrasing while implying it is yours.

Academic integrity

Academic integrity covers the commitments that make scholarly claims trustworthy: reporting honestly what you did, crediting whoever got there first, and being transparent about the help you had — from a colleague, an editor, or a tool.

Secondary source

A secondary source interprets, summarizes, or argues about evidence it did not itself generate: a review article, a textbook chapter, a critical monograph.

Write it once.
Prove you wrote it.

Copyeditor is an academic writing workspace — structured drafting, citations that build their own bibliography, and a signed record of how your document came together.

Apply for beta

*Beta opens this season — starting with students and researchers.