After an experiment with a Nominated Assembly (Barebone's Parliament), the Grandees in the Army, through the Council of State imposed a new constitutional arrangement under a written constitution called the Instrument of Government. Under the Instrument of Government executive power lay with a Lord Protector (an office to be held for the life of the incumbent) and there were to be triennial Parliaments, with each sitting for at least five months. Article 23 of the Instrument of Government stated that Oliver Cromwell was to be the first Lord Protector. The Instrument of Government was replaced by a second constitution (the Humble Petition and Advice) under which the Lord Protector could nominate his successor. Cromwell nominated his son Richard who became Lord Protector on the death of Oliver on 3 September 1658.