What is measured by a plumb line?

A plumb line is a simple yet effective tool used in construction and carpentry to determine vertical alignment. It consists of a weight suspended from a string, allowing gravity to ensure that the string hangs perfectly straight down. This vertical reference is crucial for various tasks, such as ensuring that walls are upright or that structures are built to the correct vertical standard.

When using a plumb line, the weight provides an accurate indication of where "down" is, making it an essential tool for achieving precise vertical alignment in building projects. This ensures that structures are safe, aesthetically pleasing, and aligned as intended. The other options, while relevant to construction and measurement, do not pertain to the specific function of a plumb line.