The First Presidency is the highest governing body of the Church of Jesus Christ of Latter-day Saints: the president of the church and two counsellors, occasionally more, whom he selects himself and who serve at his pleasure. It holds the executive authority of the movement. The letters that direct practice in every congregation worldwide are issued over its signatures, and on doctrine and on the calling of general officers it acts jointly with the Quorum of the Twelve Apostles.
Its most distinctive feature is that it is not a permanent institution. The First Presidency exists only while there is a president, and on his death it dissolves at once. Its counsellors, who are almost always apostles, resume their seats in the Quorum of the Twelve at their original seniority, as though the intervening years in the presidency had not occurred, and the church is governed by the Twelve until a new president is ordained and forms a new First Presidency by choosing new counsellors. A man may therefore be a counsellor in the First Presidency, return to the Twelve, and later be called back, and several have.
The arrangement is deliberate. It prevents a presidency from outliving the president who formed it, and it removes any question of a counsellor having a claim on the succession, which is settled entirely by seniority in the Twelve.