21 (b) No officer may serve on two consecutive boards under this section, if the second board considers any warrant officer who was considered by the first board. (c) The Secretary concerned shall prescribe all other matters relating to the functions and duties of the boards, including the number of members constituting a quorum. § 559. Warrant officers: eligibility for promotion Each permanent regular warrant officer shall be considered by a selection board for promotion to the next higher permanent regular warrant officer grade far enough in advance of the date upon which he will complete (1) three years of service in his permanent regular warrant officer grade, if he is in the grade of warrant officer, W - 1, or (2) six years of service in his permanent regular warrant officer grade, if he is in the grade of chief warrant officer, W-2, or chief warrant officer, W - 3, so that he may be promoted as of the day after the date on which he will complete that service, § 560. Warrant officers: promotion; selection procedure (a) The Secretary concerned shall furnish each selection board a list of all permanent regular warrant officers, in order of seniority in permanent regular grade, who are eligible to be considered for promotion to the next higher permanent regular warrant officer grade. (b) From the list of warrant officers, W - 1, the selection board shall select those whom it considers fully qualified for promotion. I t shall also report the names of those whom it recommends for termination under section 1164 of this title. (c) The Secretary concerned shall prescribe the number of warrant officers who may be selected under this section for promotion to each of the grades of chief warrant officer, W^-3, and chief warrant officer, W-4. The number so prescribed by him for a grade may not be less than 80 percent of the number of warrant officers who are being considered for the first time for promotion to that grade. (d) From the list of chief warrant officers, W-2, and chief warrant officers, W - 3, the selection board shall select for promotion to the next higher permanent regular warrant officer grade those whom it considers best qualified for promotion, but not more than the number specified by the Secretary. (e) Under such regulations as the Secretary may prescribe, the selection board shall report the names of those chief warrant officers considered by it whose records and reports establish, in its opinion, their unfitness or unsatisfactory performance in their permanent regular grades. A warrant officer whose name is so reported shall be retired, enlisted, or separated under section 1165 or 1166 of this title. (f) The names of warrant officers selected for promotion under this section shall be arranged in the board's report in order of seniority m permanent regular grade. (g) The report of the selection board shall be submitted to the Secretary concerned. The Secretary may approve or disapprove all or part of the report.