The Palácio de São Bento is the home of the Portuguese parliament. The Palace has its origin in the first Benedictine monastery of Lisbon, established in 1598. The large building, of rectangular shape, had a church flanked by two towers, four cloisters, dormitories and kitchen. From 1820 on, the old monastery was systematically adapted to its new functions and the first architect in charge was Possidónio da Silva, who designed the first session rooms. The Chapter house of the monastery was totally remodelled by French architect Jean François Colson into a session room in 1867. The Portuguese Senate used to meet in this room until the 1976 Constitution established unicameralism. Just behind the main building there is a mansion, dated in 1877 that serves as residence for the Prime Minister of Portugal. It has been the Prime Minister's official residence since 1938.