Massachusetts General Laws Part I. Administration of the Government (Ch. 1-182) Ch. 142, § 3

No person shall engage in the business as a master plumber or a master gas fitter or work as a journeyman plumber or as a journeyman gas fitter or as an apprentice plumber or as an apprentice gas fitter or as an undiluted liquefied petroleum gas installer or as a limited undiluted liquefied petroleum gas installer, nor solicit, by sign, listing or any other form of advertisement, work regulated or controlled by this chapter or by any ordinance, by-law, rule or regulation made hereunder, unless he is lawfully registered, or has been licensed by the examiners as provided in this chapter. Any person so licensed as a master plumber or a journeyman plumber may carry on the work of a gas fitter throughout the commonwealth, notwithstanding any local ordinance, by-law, rule or regulation to the contrary, and may engage in the work of installing house drainage and connecting with common sewers without being required to have any local license therefor, but shall be subject to local regulations relative to permits and bonding requirements. This section shall not apply to authorized employees of any gas company organized under chapter one hundred and sixty-four; said employees shall not require a gas fitter's license while performing work for said gas company.

The license or certificate of a journeyman plumber or a gas fitter or an apprentice plumber or a gas fitter shall be exhibited whenever required by an inspector of plumbing or gas fitting.

The license or certificate of a master plumber or a master gas fitter shall at all times be displayed conspicuously within his place of business. Any sign, listing or advertisement of a master plumber, a master gas fitter, a journeyman plumber or a journeyman gas fitter shall contain his designation and license number.