How do you check if a rule is vertical?

• A. Use of set square or protractor
• B. Plumb line
• C. Spirit Level
• D. Laser level

Answer: (B)

Gravity provides a constant reference direction: straight down. A plumb line uses that by hanging a weight on a string, which settles so the string points exactly vertical. If you align the edge of the rule with that vertical line, you know the rule is truly vertical. The other tools don’t give this gravity-defined vertical: set squares and protractors measure angles but don’t provide a true downward reference, and a spirit level is aimed at showing horizontal (level) or, when used for vertical checks, relies on different orientation. So the straightforward way to confirm verticality is to use a plumb line.