Since the Spanish Constitution was passed in 1978, Spain has been a constitutional monarchy. The country’s political structure consists of a Head of State (the king), a Prime Minister (also called the President of the Government) and a Council of Ministers. The legislative branch has two groups of mostly elected officials: the Congress of Deputies and the Senado. The headquarters for the 265 members of the Senate is the Palacio del Senado, a building at the Plaza de la Marina Espánola that was constructed in 1590 for an Augustinian convent. This modern structure by architect Salvador Gayarre was added in 1991.