22 Of the Melchizedek Priesthood, three Presiding High Priests, chosen by the body, appointed and ordained to that office, and upheld by the confidence, faith, and prayer of the church, form a quorum of the Presidency of the Church.
23 The twelve traveling councilors are called to be the Twelve Apostles, or special witnesses of the name of Christ in all the world—thus differing from other officers in the church in the duties of their calling.
24 And they form a quorum, equal in authority and power to the three presidents previously mentioned.
25 The Seventy are also called to preach the gospel, and to be especial witnesses unto the Gentiles and in all the world—thus differing from other officers in the church in the duties of their calling.
26 And they form a quorum, equal in authority to that of the Twelve special witnesses or Apostles just named.
27 And every decision made by either of these quorums must be by the unanimous voice of the same; that is, every member in each quorum must be agreed to its decisions, in order to make their decisions of the same power or validity one with the other--
28 A majority may form a quorum when circumstances render it impossible to be otherwise--
(Doctrine and Covenants | Section 107:22 - 28)