The separation of the executive and the legislature is the key difference between a presidential system and a parliamentary system. The presidential system elects a head of government independently of the legislature, while in contrast, the head of government in a parliamentary system answers directly to the legislature. Presidential systems necessarily operate under the principle of structural separation of powers, while parliamentary systems do not; however, the degree of functional separation of powers exhibited in each varies – dualistic parliamentary systems such as the Netherlands, Sweden and Slovakia forbid members of the legislature from serving in the executive simultaneously, while Westminster-type parliamentary systems such as the United Kingdom require it. Heads of government under the presidential system don't depend on the approval of the legislature as they do in a parliamentary system (with the exception of mechanisms such as impeachment).
