The straight sections of plumbing systems are called "pipes" or "tubes". A pipe is typically formed via casting or welding, whereas a tube is made through extrusion. Pipe normally has thicker walls and may be threaded or welded, while tubing is thinner-walled and requires special joining techniques such as brazing, compression fitting, crimping, or for plastics, solvent welding. These joining techniques are discussed in more detail in the piping and plumbing fittings article.
Most plumbers, pipefitters, and steamfitters learn their trade through a 4- or 5-year apprenticeship. Apprentices typically receive 2,000 hours of paid on-the-job training, as well as some classroom instruction, each year. In the classroom, apprentices learn safety, local plumbing codes and regulations, and blueprint reading. They also study mathematics, applied physics, and chemistry. Apprenticeship programs are offered by unions and businesses. Although most workers enter apprenticeships directly, some start out as helpers. The Home Builders Institute offers a pre-apprenticeship training program in plumbing and other trades.