“Q.E.F.,” sometimes written “QEF,” is an abbreviation for the Latin phrase “quod erat faciendum” (“that which was to be done”). It is a translation of the Greek words used by Euclid to indicate the end of the justification of a construction, while “Q.E.D.” was the corresponding end of proof of a theorem (cf. Heath 1956, pp. 124-129).

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