Md. Code, Business Occupations and Professions § 12–401

(a) A person may engage in the business of providing plumbing services if:

(1) the person is a master plumber or holder of a limited master plumber license; or

(2) the person:

(i) designates a master plumber or holder of a limited master plumber license who actively directs, controls, and is responsible for all plumbing services; and

(ii) otherwise provides the plumbing services in accordance with this title.

(b) A person may engage in the business of providing propane gas services if:

(1) the person is the holder of a propane gas fitter certificate; or

(2) the person:

(i) designates a holder of a propane gas fitter certificate who actively directs, controls, and is responsible for all propane gas services; and

(ii) otherwise provides the propane gas services in accordance with this title.

(c) A person may engage in the business of providing natural gas services if:

(1) the person is the holder of a master natural gas fitters license; or

(2) the person:

(i) designates a holder of a master natural gas fitters license who actively directs, controls, and is responsible for all natural gas services; and

(ii) otherwise provides natural gas services in accordance with this title.