(Pierre Mauclerc), d. 1250, duke or count of Brittany (121337). The son of Robert II, count of Dreux, he married Alix, half sister and heiress of Arthur I
duke of Brittany. His surname, meaning bad cleric, probably derived from the fact that he studied for the priesthood but abandoned it. Making peace with the church, he took part in the crusade against the Albigenses. Peter was a leader in the rebellions of the nobles against the regency of Blanche of Castile
. Upon his son's majority (1237) Peter ceded Brittany to him. Peter joined the Crusade of 1239 and later accompanied Louis IX on the Crusade to Egypt in 1248. He died on the return voyage. He is also known as Peter of Dreux.