abbreviation

QED

kyoo-ee-DEE

QED (abbreviation) means Written at the end of a mathematical proof or logical argument to mark that the thing set out to be proved has now been proved. Example: “Therefore the triangles are congruent. QED.”

abbreviation

Written at the end of a mathematical proof or logical argument to mark that the thing set out to be proved has now been proved.

"Therefore the triangles are congruent. QED."

"He filled three boards, wrote QED, and sat down."

interjection

informal Said after evidence or an example that settles an argument, often with a note of triumph.

"You said nobody would notice. Everyone noticed. QED."

How to Use QED

Learner’s notes

In plain English"There, that proves it" — written at the end of a proof.

When to use it

Technical in mathematics; playfully smug in ordinary speech.

Memory tip

Capitals with no full stops between letters is now the usual styling: QED, not Q.E.D.

Trace the full origin ↓

Easily confused with

QEF i.e. → e.g. → quantum electrodynamics (also abbreviated QED)

Common pairings

QED. write QED and QED

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Test yourself on “QED” A quick quiz — meaning, synonyms & usage →

Fill the Gap

Can you complete this real example?

Therefore the triangles are congruent. _____.

Etymology

An initialism of quod erat demonstrandum, 'which was to be demonstrated', itself a translation of the formula used by Greek mathematicians such as Euclid.

Origin: Latin

Synonyms

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∎ therefore proved

Frequently Asked Questions

What does QED stand for?

Quod erat demonstrandum, Latin for "which was to be demonstrated". It signals that the argument has arrived exactly at the statement it promised to prove.

What is the difference between QED and QEF?

QED (quod erat demonstrandum) closes a proof of a proposition. QEF (quod erat faciendum, "which was to be done") closes a construction — a geometrical task carried out rather than a truth established.

Do mathematicians still write QED?

Some do, but modern papers more often end a proof with a filled or open square, called a tombstone or Halmos symbol. QED survives mainly in teaching, in older texts and in everyday rhetorical use.

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Vocabulary lists that include QED:

Words That Start With Q