This paper analyses various systems of government understood as formal and actual relations between the legislative and executive authority. According to the principle of segregation of powers they are segregated first with regards to the scope of competence and then with regards to the competent authority, consequently allowing for a clear distinction between the legislative and executive authority, and for the setting of relations between them. Hence, the principle allows us to define two basic, or 'pure' systems of government, i. e. the parliamentary system and a contrario the presidential system. The first is characterised by a flexible distribution of powers, accompanied by the whole mechanism of various relations and subordinations between authorities. It prefers, therefore, a 'dialogue' or even a 'collaboration' between authorities, encouraging all types of contacts and links between the legislative and executive authority. The best known examples are, on one hand, the right of the parliament to adopt the vote of non-confidence in relation to the cabinet, while on the other hand, the executive authority has the right to dissolve the parliament before the term's end. The parliamentary system has been subjected to changes and modifications all the time. One can find a classic parliamentary- (a dual system), a monistic parliamentary- ( with dominance of the legislative authority) and finally, a rationalised parliamentary system, the most frequent form nowadays. It seems that the most far-reaching form of rationalisation is a semi-presidential system, called also a mixed system, a presidential/parliamentary system or a presidential/prime minister's system.The presidential system is characterised by a very strict segregation of powers, and the best example of such system can be found in the USA. However, the presidential system has not been adopted anywhere else except for the USA. in spite of many attempts of its transplantation. The far-reaching transformation of these systems of government, especially in the parliamentary system have caused that today the two systems can be rarely found in their pure form. We usually deal with some combined solutions which mix in various proportions and combinations various elements from these pure systems. Moreover, the question whether the parliamentary system is better than the presidential one has lost its relevance. What is important today is to ensure stability and efficiency of the system of government.
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