A strong string stretched taut along the planned piping route at the desired slope is called a:

The term "grade line" refers to a strong string or line that is stretched taut along the planned piping route to ensure that the pipes are installed at the correct slope for proper drainage and water flow. This line serves as a crucial reference point for installers, as it allows them to maintain consistent elevation changes throughout the installation process. Using a grade line helps ensure that the piping system functions effectively, minimizing issues such as water pooling or inadequate drainage that could arise from improper slopes.

In plumbing, understanding the concept of a grade line is essential since proper slope is critical for the functioning of sewer and drainage systems. Other terms like "level line," "reference line," or "check line" may pertain to other contexts or measurements, but the specific role of establishing the slope in piping applications is best described by the term "grade line."