The Jus Exclusivæ meaning right of exclusion; sometimes called the papal veto, was the right claimed by several Catholic monarchs of Europe to veto a candidate for the papacy.
At times the right was claimed by the French monarch, the Spanish monarch, the Holy Roman Emperor, and the Emperor of Austria. These powers would make known to a papal conclave, through a crown-cardinal, that a certain candidate for election was considered objectionable as a prospective Pope.
This right seems to have been claimed during the 17th century. It does not seem to be related to a right exercised by Byzantine emperors and Holy Roman Emperors to confirm the election of a Pope, which was last exercised in the Early Middle Ages.
The Jus Exclusivæ was first exercised by Spain in 1644 conclave to exclude Cardinal Sacchetti. The conclave eventually elected Cardinal Giovanni Battista Pamphili, who became Pope Innocent X. Fortunately for Pamphili, Cardinal Jules Mazarin of France arrived too late to present the veto of France at the conclave against Pamphili, who had already been officially elected.
At the 1903 conclave, Cardinal Rampolla was the clear favourite and seemed a few electoral moments away from ascending St Peter’s throne. However, the Jus Exclusivæ was exercised by Cardinal Jan Puzyna de Kosielsko from Krakow in the name of Emperor Franz Joseph in a very late stage of the proceedings. Much of the conclave objected the veto, but it was reluctantly accepted. Is was the last time the Jus Exclusivæ was exercised. The 1903 conclave eventually elected Cardinal Guiseppe Sarto who became Pope Pius X, later to be confirmed as a saint.