法國於 2000 年及 2008 年進行兩次重要憲法修正，以便從制度面降低左右共治的機率。然而左右共治情形仍有可能發生，而共治時的「保留權限」更是雙行政首長依法據理力爭的場域。法國自從 1986 年在國會大選後產生與總統政治理念不同的多數黨，造成分屬左右兩大黨派的總統／總理同時存在的局面，15年來共發生 3 次（1986 至 1988 年，1993 至 1995 年，1997 至 2002 年）。根據在憲法及相關規定，總統及總理在國防及外交方面的權限規範模糊且重疊，在實踐上也不具一致性。一方面，國防及外交權傳統被視為是總統的「保留範圍」(domaine reservé），另一方面，按第五共和憲法規定，國防及外交政策是總統及總理的「共享範圍」，左右共治未出現前，因總統的強勢領導及總統／總理屬同一政黨，且在總統掌握國會多數及總理任命權等政治現實下，憲法雖然賦予總理在國防及外交政策的若干權力，總理事實上並未充分行使；直到左右共治局面出現，擁有巨大政治實力的總理要求行使國防及外交政策決定權的法理空間，而總統亦提出遭擱置多年的「保留範圍」法理依據以爭取主導國家對外政策的實權，因而在實踐上有許多磨合及衝突之處。在探討「保留範圍」的規範與三次左右共治時期的實踐特色後，結論將提出幾點心得。

Cohabitation became a part of French political life during 1986-1988, 1993-1995, 1997-2002, when the President of the Republic is from a different political party than the actual majority of the members of parliament, and because of the duality of the executive: an elected President and a premier acceptable both to the President and to the legislature. Cohabitation was a product of the Constitution of the Fifth Republic, while according to the Constitution, the President governs the state, the prime minister similarly directs the work of government, providing the PM a strong leadership to the administrative and legislative branch. In practice, because of the "domaine reservé", granted by the Constitution to the President in the policy making of the national defense and foreign affaires, the oscillation and the conflict of powers between the President and Prime Minister were evident. During the three Cohabitations, the President retains his traditional privileged status, foreign affaires and European policy are shared between the President as head of state and the PM as a head of the government, while national defence, nuclear strategy remained the exclusive preserve of the President.