one tier board. The United Kingdom, the United States, and most Commonwealth countries have single unified boards of directors. In Germany, companies have two tiers, so that shareholders (and employees) elect a "supervisory board", and then the supervisory board chooses the "management board". There is the option to use two tiers in France, and in the new European Companies (Societas Europaea). Recent literature, especially from the United States, has begun to discuss corporate governance in the terms of management science. While post-war discourse centred on how to achieve effective "corporate democracy" for shareholders or other stakeholders, many scholars have shifted to discussing the law in terms of principal–agent problems. On this view, the basic issue of corporate law is that when a "principal" party delegates his property (usually the shareholder's capital, but also the employee's labour) into the control of an "agent" (i.e. the director of the company) there is the possibility that the agent will act in his own interests, be "opportunistic", rather than fulfill the wishes of the principal. Reducing the risks of this opportunism, or the "agency cost", is said to be central to the goal of corporate law. Constitution The rules for corporations derive from two sources. These are the country's statutes: in the US, usually the Delaware General Corporation Law (DGCL); in the UK, the Companies Act 2006 (CA 2006); in Germany, the Aktiengesetz (AktG) and the Gesetz betreffend die Gesellschaften mit beschränkter Haftung (GmbH-Gesetz, GmbHG). The law will set out which rules are mandatory, and which rules can be derogated from. Examples of important rules which cannot be derogated from would usually include how to fire the board of directors, what duties directors owe to the company or when a company must be dissolved as it approaches bankruptcy. Examples of rules that members of a company would be allowed to change and choose could include, what kind of procedure general meetings should follow, when dividends get paid out, or how many members (beyond a minimum set out in the law) can amend the constitution. Usually, the statute will set out model articles, which the corporation's constitution will be assumed to have if it is silent on a bit of particular procedure. The United States, and a few other common law countries, split the corporate constitution into two separate documents (the UK got rid of this in 2006). The memorandum of association (or articles of