What is the defining characteristic of a horizontal pipe?

The defining characteristic of a horizontal pipe is that it is installed with an angle of less than 45 degrees relative to the horizontal plane. This means the pipe runs mostly parallel to the ground, facilitating the flow of fluids with minimal resistance due to gravity. Pipes that are level or have a slight slope can effectively transport water or waste, which is essential in plumbing systems for proper drainage and water supply.

Although options like being "always straight" may suggest the straightness of many horizontal pipes, this does not capture their fundamental definition. Pipes can have bends or curves while still being horizontal as long as the overall angle remains within the criteria. The assertion that horizontal pipes are "used solely for drain pipes" is too restrictive, as they can also be present in water supply lines, while the option stating that it is the "main water supply line" does not accurately describe the characteristic orientation of horizontal piping.