The Counselors are customarily chosen from among the Quorum of the Twelve Apostles, and work closely with the Church President in guiding the entire Church. In the case of an infirm president they may be called upon to perform his duties in his absence, and if need be additional Counselors, who are not numbered, are called to assist them. Like the Church President and President of the Quorum of the Twelve Apostles, Counselors in the First Presidency are known as "President" (thus the current First Presidency are referred to as President Gordon B. Hinckley, President Thomas S. Monson, and President James E. Faust).
Counselors serve until either their own deaths or the death of the Church President who appointed them,whichever comes first...the death of a Church President dissolves the First Presidency, and leaves the President of the Quorum of the Twelve Apostles as the senior leader of the Church, until he names new Counselors to reconstitute the First Presidency with himself as Church President at a time of his choosing.
Customarily the counselors of the late president are retained, but a notably infirm Counselor may revert to his place among the Twelve Apostles with a healthier man called as Counselor in his place. On the death or release of a First Counselor, the Second Counselor usually succeeds and a new Second Counselor is named.