The Pactum Lotharii is an agreement signed February 23 840 between Venice and the Carolingian Empire, during the respective governments of Pietro Tradonico and Lothair.
This document was one of the first acts that testify to the separation between the nascent Republic of Venice and the Byzantine Empire: for the first time the Doge, on its own initiative, undertook agreements with the Western world at the time.
However, the pact was more of a symbolic value because it reiterated the agreements already made in the past between the two empires. It concerned the rights of land use and administration of justice.
The Pactum Lotharii is also a valuable document that allows to know precisely the territory of the ancient Venetian ducat. The boundaries thus coincided with the old limit of the lagoons (well most extensive of current) and the mainland reached even the Abbey of St. Hilary and the area of the ancient Altinum.