The term 'Principal Pipes' refers to a job in the plumbing industry. The job involves overseeing the installation and maintenance of pipes, fixtures, and other plumbing systems. The job title is often abbreviated to 'MAINS, ' which is a five-letter acronym for 'Master of All Installations and Networks.' This job requires a great deal of knowledge and experience in the plumbing field, as well as the ability to manage a team of workers.