Each building must be separately and independently connected with a public or private sewer, or cesspool, except where a building is located in the rear of the same lot with another building, when its plumbing and drainage system may be connected to the house-drain of the front building behind the house trap and fresh air inlet which shall be used for both buildings if sewer connected; or may be connected to an existing cesspool of front house and be provided with a separate house trap and fresh air inlet. FURTHER PROVISION Where there is no sewer in the street or avenue, and it is possible to construct a private sewer to connect in an adjacent street or avenue, a private sewer must be constructed. It must be laid outside the curb, under the roadway of the street. All pipes and traps should, where possible, be exposed to view. They should always be readily accessible for inspection and repairing. In every building where there is a leader connected to the drain, if there are any plumbing fixtures, there must be at least one 4″ pipe extended above the roof for ventilation. The contents of settling chamber or dust receptacle for vacuum cleaners may be discharged into a plumbing and drainage system. Leaders must be trapped with cast-iron running traps so placed as to prevent freezing. Rain-water leaders must not be used as soil, waste or vent pipes, nor shall any such pipe be used as a leader. To have an intelligent understanding of what the plumber has to know, it might be well to know what certain terms are which are used in the plumbing rules. DEFINITIONS The term “private sewer” is applied to main sewers that are not constructed by and under the supervision of the Department of Public Works. The term “house sewer” is applied to that part of the main drain or sewer extending from a point two feet outside of the outer front wall of the building, vault or area to its connection with public sewer, private sewer or cesspool. The term “house drain” is applied to that part of the main horizontal drain and its branches inside the walls of the building, vault or area and extending to and connecting with the house sewer. The term “soil line” is applied to any vertical line of pipe having outlets above the floor of first story for water closet connections. The term “waste line” is applied to any vertical line of pipe having outlets above the first floor for fixtures other than water closet. The term “vent pipe” is applied to any special pipe provided to ventilate the system of piping and to prevent trap siphonage and back pressure. THE TRAP Most important from the hygiene point of view is the trap, which is a curved pipe permitting the last of a flow of water to remain in the pipe to prevent a back flow of sewage gas into the house. There are for use various forms of traps under different circumstances which, of course, are entirely the plumber’s business.