René-Prosper Tassin

United in close friendship with his brother in religion, Charles-François Toustain, he collaborated with him on a new edition of the works of Theodore the Studite, which task led them to visit Rome together.

Their work was interrupted by a dispute between the Benedictine Abbey of St. Ouen and the chapter of Rouen, which was supported by the erudite Jean Saas.

To defend the authenticity of the deeds of their abbey they were obliged to make a deep study of diplomatics, dealing with diplomas, charters, and other official documents, for which Jean Mabillon had laid the foundations in his great Latin work, De re diplomatica (1681).

As a result of their researches they wrote the Nouveau traité de diplomatique, in six quarto volumes, which appeared between the years 1750 and 1765.

Tassin later wrote his Histoire littéraire de la Congrégation de Saint-Maur (Paris and Brussels, 1770), a model history containing the lives and list of works, printed or in manuscript, of all the learned authors of the Congregation, from its formation in 1618 until his own time, including a list of their works, printed or in manuscript.