Md. Const. art. XI-F, § 5

The General Assembly, by law, shall classify all code counties by grouping them into not more than four classes based either upon population as determined in the most recent Federal or State census or upon such other criteria as determined by the General Assembly to be appropriate. Not more than one such grouping of code counties into four (or fewer) classes may be in effect at any one time, and the enactment of any grouping of code counties into four (or fewer) classes repeals any other such grouping then in effect. Code counties may be classified only as provided in this section.