Gov. Code, § 29910.1

The board may divide the principal amount of any issue into two or more series and fix different dates for the bonds of each series. The bonds of one series may be made payable at different times from those of any other series; provided, that the earliest maturity of each issue or series, as the case may be, shall not be more than two years from the date of the bonds of said issue or series. The final maturity date of any bond shall not exceed 40 years from the date of the bond. Every year beginning with the date of the earliest maturity of each issue or series of bonds, as the case may be, not less than one-fortieth of the whole of the indebtedness evidenced by such issue or such series shall be payable; provided, however, the bonds of any issue or series irrespective of the purpose for which the same are to be issued may be made to mature and become payable in approximately equal total annual installments of interest and principal, during the term of the bonds computed from the first year in which any part of the principal shall mature to the date of final maturity which annual installments may vary one from another in amounts not exceeding in any year more than 5 percent of the total principal amount of the bonds of such issue or of the series thereof then proposed to be issued.