Maryland Code, Business Occupations and Professions § 12-307

(a) While a master plumber license is in effect, it authorizes the licensee to provide plumbing services.

(b) While a journey plumber license is in effect, it authorizes the licensee to provide plumbing services only under the direction and control of a master plumber or a holder of a limited master plumber license.

(c) While an apprentice plumber license is in effect, it authorizes the licensee to assist in providing plumbing services only under the direction and control of a master plumber or a holder of a limited master plumber license.

(d) While a propane gas fitter certificate is in effect, it authorizes the certificate holder to provide propane gas services and to directly supervise others in the provision of those services in every political subdivision of the State.

(e) While a master natural gas fitters license is in effect, it authorizes the licensee to provide natural gas services.

(f) While a journeyman natural gas fitters license is in effect, it authorizes the licensee to provide natural gas services only under the direction and control of a licensed master natural gas fitter.

(g) While an apprentice natural gas fitters license is in effect, it authorizes the licensee to provide natural gas service only under the direction and control of a licensed master natural gas fitter.