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Separation of powers, a term coined by French political Enlightenment thinker Baron de Montesquieu have pointed out that, regardless of whether it accomplishes this end, it also slows down the process of governing, promotes executive dictatorship and unaccountability, and tends to marginalize the legislature.
No democratic system exists with an absolute separation of powers or an absolute lack of separation of powers. Nonetheless, some systems are clearly founded on the principle of separation of powers, while others are clearly based on a mingling of powers.

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Political parties Montesquieu's tripartite system
In democratic systems of governance, a continuum exists between "Presidential government" and "Parliamentary government". "Separation of powers" is a feature more inherent to presidential systems, whereas "fusion of powers" is characteristic of parliamentary ones. "Mixed systems" fall somewhere in between, usually near the midpoint; the most notable example of a mixed system is France's (current) Fifth Republic.
In fusion of powers, one branch (invariably the elected legislature) is supreme, and the other branches are subservient to it. In separation of powers, each branch is largely (although not necessarily entirely) independent of the other branches. Independent in this context means either that selection of each branch happens independently of the other branches or at least that each branch is not beholden to any of the others for its continued existence.
Accordingly, in a fusion of powers system such as that of the United Kingdom, first described as such by Walter Bagehot, the people elect the legislature, which in turn "creates" the executive. As Professor Cheryl Saunders writes, "...the intermixture of institutions [in the UK] is such that it is almost impossible to describe it as a separation of powers." of the executive; instead, the executive is chosen by other means (direct popular election, electoral college selection, etc.) In a parliamentary system, when the term of the legislature ends, so too may the tenure of the executive selected by that legislature. Although in a presidential system the executive's term may or may not coincide with the legislature's, her selection is technically independent of the legislature. However, when the executive's party controls the legislature, the executive often reaps the benefits of what is, in effect, a "fusion of powers". Such situations may thwart the constitutional goal or normal popular perception that the legislature is the more democratic branch or the one "closer to the people," reducing it to a virtual "consultative assembly," politically or procedurally unable—or unwilling—to hold the executive accountable in the event of blatant, even boldly admitted, "high crimes and misdemeanors."

Separation of powers vs. fusion of powers

Other branches
With the title Comptroller General, Auditor General or Comptroller and Auditor General, the European Union's Court of Auditors and Taiwan's Control Yuan are individual or bodies of independent ombudsmen. They are often independent of the other branches of government.
Their purpose is to audit government expenditure and general activity.

Auditory
Sun Yat Sen proposed a branch of government based on the Imperial examination system used in China. The "Examination Yuan" (Traditional Chinese: 考試院; pinyin: Kǎoshì Yuàn), as it is called in Taiwan, is in charge of validating the qualification of civil servants. This structure has been implemented in the Republic of China.

Civil examination
Costa Rica's Supreme Elections Tribunal is a branch of government that manages elections. Similar independent institutions exist in many other democratic countries, however they are not seen as a branch of government. In many countries, these are known as Electoral Commissions.

Electoral
Many philosophers and political scientists believe that democratic governments are created and constitutions exist to serve the people. The people have their own system of checks and balances by electing the legislative and executive branches. The government also draws its power directly from the people. Without the people, there is no government, just as without the legislative branch, there can be no judicial branch.
In the Constitution of Venezuela, the "citizen's power" is a formal branch of government, though it acts like auditors' branches in other jurisdictions.
See also:

Direct Democracy
Initiative
Referendum
Recall election The people
The federal executive of the United States is a very large bureaucracy, and due to civil service rules, most middle- and low-level government workers do not change when a new President is elected. (New high-level officials are usually appointed and must be confirmed by the Senate.) Moreover, semi-independent agencies (such as the Federal Reserve or the Federal Communications Commission) may be created within the executive by the legislature. These agencies exercise legally defined regulatory powers. High-level regulators are appointed by the President and confirmed by the legislature; they must follow the law and certain lawful executive orders. But they often sit for long, fixed terms and enjoy reasonable independence from other policy makers. Because of its importance to modern governance, the regulatory bureaucracy of the executive is sometimes referred to as a "fourth" branch of government.
This separation is even more pronounced in the United Kingdom. The separation was a prominent element of the Yes Minister comedy television series.

Independent executive agencies

U.S. contractors becoming a fourth branch of government by Scott Shane and Ron Nixon
The Fifth Branch of Government by Joe Wortham The press

Main articles: Freedom of the press and public broadcasting The press around the world
Constitutions with a high degree of separation of powers are found worldwide. The UK system is distinguished by a particular entwining of powers. India's democratic system also offers a clear separation of power under Lok Sabha (lower house of parliament), Rajya Sabha (upper house of Parliament), and the President of India, who overlooks independent governing branches such as the Election commission and the Judiciary. Under the Indian constitution, just as in the British system, the Prime Minister is a head of the governing party and functions through a selected group of ministers. In Italy the powers are completely separated, even if Council of Ministers need the vote of confidence from both chambers of Parliament, that's however formed by a wide number of members (almost 1,000).
Countries with little separation of power include New Zealand and Canada. Canada makes limited use of separation of powers in practice, although in theory it distinguishes between branches of government.
Complete separation-of-powers systems are almost always presidential, although theoretically this need not be the case. There are a few historical exceptions, such as the Directoire system of revolutionary France. Switzerland offers an example of non-Presidential separation of powers today: It is run by a seven-man executive branch, the Federal Council. However, some might argue that Switzerland does not have a strong separation of powers system, as the Federal Council is appointed by parliament (but not dependent on parliament), and the judiciary has no power of review.

Various models around the world

Main article: Separation of powers in Australia Australia: three branches

Main article: Government of the People's Republic of China People's Republic of China
After eight years of social conflict, the question of who would lead Costa Rica and which transformationist model the State would use was decided by who killed the president. A constituent assembly followed and drew up a new constitution, approved in 1949. This document was an edit of the constitution of 1871, as the constituent assembly rejected more radical corporativist ideas proposed by the ruling junta. Nonetheless, the new constitution increased centralization of power at the expense of municipalities and eliminated provincial government altogether.
It established the three supreme powers as the legislature, executive, and judicial branches, but also created two other autonomous state organs that have equivalent power but not equivalent rank. The first is the Supreme Elections Tribunal (electoral branch) which controls elections and makes unique, unappealable decisions on their outcomes.
The second is the office of the Comptroller General (auditory branch), an autonomous and independent organ nominally subordinate to the unicameral legislative assembly. All budgets of ministries and municipalities must pass through this agency, including the execution of budget items such as contracting for routine operations. The Comptroller also provides financial vigilance over government offices and office holders, and routinely brings actions to remove mayors for malfeasance, firmly establishing this organization as the fifth branch of the Republic.

Costa Rica: five branches
The five institutions (in four branches) of the European Union are:

European Commission - executive
European Parliament & Council of the European Union - legislative
European Court of Justice - judicial
European Court of Auditors - auditory European Union: four branches

Main article: Government of France France
The six main bodies enshrined in the Basic Law for the Federal Republic of Germany are:
There is also a judicial branch made up of five supreme courts, state (lander) based courts beneath them, and a rarely used senate of the supreme courts.

Federal President (Bundespräsident)
Federal Cabinet (Bundesregierung) - executive
Federal Diet (Bundestag) & Federal Council (Bundesrat) - legislative
Federal Assembly (Bundesversammlung) - presidential electoral college
Federal Constitutional Court (Bundesverfassungsgericht) Germany: six branches

Main article: Government of Japan Japan
Some countries take the doctrine further than the three-branch system. The politics of Taiwan, for example, has five branches: the Executive Yuan, Legislative Yuan, Judicial Yuan, Control Yuan (auditory branch), and Examination Yuan.
Due in part to the Republic's youth, the relationship between its executive and legislative branches are poorly defined. An example of the problems this causes is the near complete political paralysis that results when the president, who has neither the power to veto nor the ability to dissolve the legislature and call new elections, cannot negotiate with the legislature when his party is in the minority. [2]

Taiwan: five branches
In parliamentary systems, a separation of powers is either unclear or even nearly non-existent . For example, in the United Kingdom, the executive forms a subset of the legislature, as does—to a lesser extent—the judiciary. The Prime Minister, the chief executive, must by convention be a Member of the House of Commons and can effectively be removed from office by a simple majority vote. Furthermore, while the courts in Britain are undoubtedly amongst the most independent in the world, the Law Lords, who are the final arbiters of judicial disputes in the UK, sit simultaneously in the House of Lords, the upper house of the legislature, although this arrangement will cease in 2009 when the Supreme Court of the United Kingdom comes into existence. Furthermore, because of the existence of Parliamentary sovereignty, while the theory of separation of powers may be studied in Britain, a system such as that of the UK is more accurately described as a "fusion of powers."
The development of the British constitution, which is not written down in one document, is based on this fusion in the person of the Monarch, who has a formal role to play in the legislature (Parliament, which is where legal and political sovereignty lies, is the Crown-in-Parliament, and is summoned and dissolved by the Queen who must give her Royal Assent to all Bills so that they become Acts), the executive (the Queen appoints all ministers of Her Majesty's Government, who govern in the name of the Crown) and the judiciary (the Queen, as the fount of justice, appoints all senior judges, and all public prosecutions are brought in her name).
The British legal code is based on common law traditions which requires:

Police or regulators cannot initiate complaints under criminal law but can only investigate, which prevents selective enforcement, e.g. the 'fishing expedition' which is often specifically forbidden
Prosecutors cannot withhold evidence from attorneys for the defendant, to do so results in mistrial or dismissal, accordingly their relation to police is no advantage
Defendants convicted can appeal, but no new evidence can usually be introduced, restricting the power of the court of appeal to the process of law applied United Kingdom
The United Nations has five principle organs [3]. These are:
The members of the councils are either or both elected by the General Assembly and determined by the UN Charter.

General Assembly - legislative
Security Council
Economic and Social Council
International Court of Justice - judicial
Secretariat - executive United Nations: five branches

Main article: Separation of powers under the United States Constitution United States: three branches
To prevent one branch from becoming supreme, and to induce the branches to cooperate, governance systems employing a separation of powers typically are created with a system of "checks and balances", a term which, like separation of powers itself, is generally credited to Montesquieu. Checks and balances refers to the various procedural rules that allow one branch to limit another, such as the authority of the president to veto legislation passed by Congress, or the power of Congress to alter the composition and jurisdiction of the federal courts.

Writes and enacts laws
Enacts taxes, authorizes borrowing, and sets the budget
Usually has sole power to declare war
May start investigations, especially against the executive branch
Often appoints the heads of the executive branch
Sometimes appoints judges
Ratifies treaties
Sometimes may veto laws
May refuse to enforce certain laws (risking impeachment by the legislature)
May refuse to spend money allocated for certain purposes
Wages war (has operational command of the military)
Makes decrees or declarations (for example, declaring a state of emergency) and promulgates lawful regulations and executive orders
Often appoints judges
Has power to grant pardons to convicted criminals
Determines which laws apply to any given case
Determines whether a law is unconstitutional
Has sole power to interpret the law and to apply it to particular disputes
May nullify laws that conflict with a more important law or constitution
Determines the disposition of prisoners
Has power to compel testimony and the production of evidence
Enforces uniform policies in a top-down fashion via the appeals process, but gives discretion in individual cases to low-level judges. (The amount of discretion depends upon the standard of review, determined by the type of case in question.)
May rule only in cases of an actual dispute brought between actual petitioners
Polices its own members
Is frequently immune to arbitrary dismissal by other branches Checks and balances
The theoretical independence of the executive and legislative branches is partly maintained by the fact that they are separately elected and are held directly accountable to the public. There are also judicial prohibitions against certain types of interference in each others' affairs. (See "separation of powers" cases in the List of United States Supreme Court cases.) Judicial independence is maintained by life appointments of judges, with voluntary retirement, and a high threshold for removal by the legislature. In recent years, there have been accusations that the power to interpret the law is being misused (judicial activism) by some judges in the US. In the checks and balances system, the judicial branch has the right to say that something is unconstitutional, like a law or a bill.
The legal mechanisms constraining the powers of the three branches depend a great deal on the sentiment of the people. A common perception is that popular support establishes legitimacy and makes possible the actual implementation of legal authority. National crises (such as the Civil War, the Great Depression, pre-Pearl Harbor World War II, the Vietnam War) have been the times at which the principle of separation of powers has been most endangered, either through official "misbehavior" or through the willingness of the public to sacrifice such principles if more pressing problems are solved. The system of checks and balances is also self-reinforcing. Potential abuse of power may be deterred, and the legitimacy and sustainability of any power grab is hindered by the ability of the other two branches to take corrective action; though they still must actually do so, therefore accountability is not automatic. This is intended to reduce opportunities for tyranny sometimes.
However, as James Madison wrote in Federalist No. 51 regarding the ability of each branch to defend itself from actions by the others, "But it is not possible =] to give to each department an equal power of self-defense. In republican government, the legislative authority necessarily predominates." Bicameralism was, in part, intended to reduce the relative power of the legislature by turning it against itself, by having "different modes of election a different principles of action." (This is one of the arguments against the popular election of Senators, which was instituted by the Seventeenth Amendment.) But when the legislature is unified, it can obtain dominance over the other branches.

Maintaining balance
The American states mirror the executive/legislative/judicial division of the federal government. Major cities tend to do so as well, but the arrangements of local and regional governments vary widely. Because the judicial branch is often a part of a state or county government, the geographic jurisdiction of local judges is often not coterminous with municipal boundaries.
In many American states and local governments, executive authority and law enforcement authority are separated by allowing citizens to directly elect public prosecutors (district attorneys and state attorneys-general). In some states, judges are also directly elected.
Many localities also separate special powers from their executive and legislative branches through the direct election of sheriffs, school boards, transit agency boards, park commissioners, etc.
Juries (groups of randomly selected citizens) also have an important role in the checks-and-balances system. They have the sole authority to not only determine the facts in most criminal and civil cases, but to judge the law, acting as a powerful buffer against arbitrary enforcement by the executive and judicial branches. In many jurisdictions they are also used to determine whether or not a trial is warranted, and in some places Grand Juries have independent investigative powers with regard to government operations.

State and local governments

Main article: Constitution of Venezuela Criticisms

Federalism, also known as vertical separation of powers - Prevents abuse by dividing governing powers, usually by separating municipal, provincial, and national governments. See also subsidiarity.
Rule of law - Prevents arbitrary exercise of the executive power, preserves general and minority rights, and promotes stability and predictability.
Democracy and civil society - Attempts to constrain elected branches of government to act in the public interest, not in self-interest.
Separation of church and state or Laïcité - Ensures freedom of religion by preventing government interference in its practice. Also constrains the power of government by maintaining freedom of conscience and belief.
Civilian control of the military - Helps prevent dictatorship that otherwise might occur through military rule.
In some systems, an independent central bank.
Separation of duties in organizations.
Independent Civil Service.
Negarchy. Notes

Absolute power
Balance of power
Corruption Perceptions Index - Parliamentary systems are, in general, perceived as less corrupt than other systems
Judicial activism
List of democracy and elections-related topics
Signing statement
Unitary executive theory
Fifth power