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In political systems based on the principle of separation of powers, authority is distributed among several branches (executive, legislative, judicial)--an attempt to prevent the concentration of power in the hands of a single group of people. In such a system, the executive does not pass laws (the role of the legislature) or interpret them (the role of the judiciary). Instead, the executive enforces the law as written by the legislature and interpreted by the judiciary. The executive can be the source of certain types of law, such as a decree or executive order. Executive bureaucracies are commonly the source of regulations.
In parliamentary systems, the executive is responsible to the elected legislature, i.e. must maintain the confidence of the legislature (or one part of it, if bicameral). The legislature can, in certain circumstances (varying by state), express its lack of confidence in the executive, which causes either a change in governing party or group of parties or a general election. Parliamentary systems have a head of government (who leads the executive, often called ministers) normally distinct from the head of state (who continues through governmental and electoral changes). In the Westminster type of parliamentary system, the principle of separation of powers is not as entrenched as in some others. Members of the executive (ministers), are also members of the legislature, and hence play an important part in both the writing and enforcing of law.
In this context, the executive consists of a leader(s) of an office or multiple offices. Specifically, the top leadership roles of the executive branch may include: