quod erat faciendum

Latin phrase

quod erat fa·​ci·​en·​dum ˌkwȯd-ˈer-ˌät-ˌfä-kē-ˈen-ˌdu̇m 

: which (is what) was to be done (originally)

—abbreviation q.e.f.

—used at the end of a geometrical construction