The Nassau Family Pact (German: Nassauischer Erbverein) was a mutual pact of inheritance and succession made in 1783 by princes of the old German noble and sovereign family of Nassau. It confirmed Salic Law to operate in favor of all agnatic lines of the family, divided into first surviving lines which existed in the Middle Ages (Walramian and Ottonian). The pact chiefly provided that in case of one of the lines becoming extinct, the other would succeed in its hereditary Nassau lands ("the main concept of the Erbverein was that if either the Ottonian or Walramian male line would become extinct the other line would succeed").
There was a clause to provide for a so-called Semi-Salic continuation to the dynasty in an undefined way if both the lines were to die out in the male line ("also arranged for that in the absence of all male successors, females could succeed"). In case of the extinction of all male lines, the closest heir to the last male will succeed and in turn will be succeeded by the heirs of that closest one. If the closest heir happens to be a woman, the pact was silent about whether her husband receives rights or not. There was no precise stipulation what precisely happens after that closest heir: will the succession evolve to heirs general, or only to heirs male; what happens if that issue dies out. However, it is easy to understand that in case of total extinction of an heir's line, the next heir (or line) will take its place.
The pact was agreed to be applied to "Imperial fiefs" which meant those territories owned or acquired in the then Holy Roman Empire. The Pact thus in 1890 determined the succession of the Grand Duchy of Luxembourg, a territory acquired into the dynasty only after the pact was sealed but which at the time had been a member of the German Confederation, a body that was regarded as the successor to the Holy Roman Empire. The pact did not apply to the succession in the Kingdom of the Netherlands, a state not then regarded as formerly imperial. Luxembourg was thus inherited by the Weilburg branch, the only extant branch from that date onwards.
In 1907, the Grand Duke of Luxembourg (William IV), head of the House of Nassau, determined that the branch of the Count of Merenberg was, as morganatic, incapable to succeed to the sovereignties of the House of Nassau. This effectively meant that the Grand Duke himself was then the only surviving agnate of the House.
Having himself only daughters, he felt the need to organize the succession further and remedy some of the undefined points. In April 1907 the grand duke decreed (approved in July 1907 by legislature of Luxembourg and thereafter enacted) amendments to the house law of Nassau. The succession law thus amended governs the current succession in Luxembourg, and apparently the succession specifically provided by the Pact itself is fulfilled and the pact's impact is exhausted.
Marie-Adélaïde succeeded according to the 1907 law, an outcome that was identical with the stipulations of this pact.
Were any successions of the House of Nassau outside Luxembourg to need to be adjudicated afterwards, it is unclear what the pact would provide — whether a line identical with that of modern Luxembourg's, or different.