Actually, in most of the States we've worked in, the plumbing trade is licensed in stages. Baby plumbers get apprentice cards for four to eight thousand working hours, the hours are signed off on by the responsible Master plumber before State testing for a Journeyman license, and then a given number of work hours are required as a Journeyman to test at the State for a Master's license. Normally, there is but one Master per plumbing company, and he's generally not the fellow crawling under your sink. In fact, he's often the owner of the Plumbing Company. Various other licensing levels exist (tradesman, irrigation specialist, fire protection, water well, etc.) and each State has it's own Plumbing Board and unique licensing requirements.