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Q. E. D. is an abbreviation of the Latin phrase "quod erat demonstrandum" (literally, "which was to be proved"). Q.E.D. may be written at the end of mathematical proofs to show that the result required for the proof to be complete has been obtained. It is not seen as frequently now as in earlier centuries.

End-of-proof symbolism in the present day is often the symbol ■ (solid black square) called the tombstone or halmos (after Paul Halmos who pioneered its use). The tombstone is sometimes open; □ (hollow black square).

In Hong Kong, students jokingly reinterpret Q.E.D. as "Question Easy Done" (in Chinglish) after they finish a "difficult" mathematical proof in their schoolwork.

Not to be confused with QED, the acronym for quantum electrodynamics.