國家元首(Head of state)是一個國家在實際上或形式上對內對外的最高代表，在國家機構體系中實質上或象徵性地居於首腦地位。當兩個或兩個以上被國際承認的主權國家，共同擁有同一位國家元首時，這種特殊的國與國關係稱為共主邦聯，又可譯作君合國、身合國或人合國(Personal union)。由於政治、法律、人民訴求的差異，有些未經法律手段定義的共主邦聯，成員之間保持著很鬆散的關係，有時彼此關係甚至還不及國際聯盟。共主邦聯依其形式而有不同狀況，有些維持各自獨立的鬆散結合，有些最終則結合成為一個聯邦制國家，也有完全解散的例子。進入20世紀，共主邦聯也越來越少見。現存最大的共主邦聯，是由英國和15個大英國協王國所組成，這些國家共同推舉英國女王伊莉莎白二世為國家元首。本文旨在探討共同元首國家在國際法上的關係，並以英國與澳紐加拿大等國為例，針對其相互關係與實際運作狀況加以探討。

The head of state is the chief representative of a state to its people and to other countries. This constitutional position is endowed with symbolic and/or actual power. A personal union is a special state-to-state relationship when two or more sovereign states share a single monarch. Personal unions vary with circumstances due to particular political and legal traditions as well as different demands and mandates from the people. Certain personal unions without legal delineation actually function as loose associations, in some instances with less internal cohesion than that of international coalitions. Some personal unions remain loose associations of independent states, some coalesce into federations, and still others eventually dissolve. Personal unions are a dying species since the Twentieth Century. The largest personal union is the Commonwealth Realms, composed of the United Kingdom and fifteen other members of the Commonwealth that have Queen Elizabeth II as the reigning monarch. This article studies the interrelations of the United Kingdon, Australia, New Zealand, and Canada to examine the legal implications of separate states sharing a single monarch.