Upon the death of the President of the Church, the First Presidency automatically dissolves, leaving the Twelve Apostles as the highest leadership group and their President as the highest official. On the death of Joseph Smith, Jr this position was held by Brigham Young, and he persuaded the Church that Smith's death left him and not Sidney Rigdon, who had been Smith's First Counselor in the First Presidency, as the senior leader...Smith had said, "Where I am not,there is no First Presidency over the Twelve."
In 1847 Young reconstituted the First Presidency, as second President of the Church, and during his tenure formalized that the succession to the Presidency of the Twelve is strictly by continuous service as an Apostle since being ordained as one of the Twelve. The original apostles of 1835 had been ranked by age, and two of them excommunicated and restored to fellowship. With this rule in place, it was John Taylor who led the church after Young's death in 1877, first as President of the Twelve and after 1880 as President of the Church with Wilford Woodruff as President of the Twelve.
Woodruff did not reorganize the First Presidency until 1889, after Taylor died in 1887, but before his death in 1898 he advised the President of the Twelve, Lorenzo Snow, "Do not wait, Brother Snow, but proceed at once to reorganize the First Presidency." Snow followed this advice and since then every interval between Presidents of the Church has been under two weeks,only long enough for the President of the Twelve to decide who his counselors in the First Presidency should be. The next Apostle in seniority after the President of the Twelve becomes the next President of the Twelve, and if he is one of the Counselors in the First Presidency, the most senior Apostle not in the First Presidency is named Acting President of the Quorum of the Twelve.
The Acting President title was first used in 1918 for Rudger Clawson, and first used simply because of the infirmity of a President of the Twelve for Howard W. Hunter in 1985-88.