What tool is used to mark absolute vertical when suspended from a string?

A plumb bob is the tool specifically designed for determining absolute vertical alignment when suspended from a string. It consists of a pointed weight attached to a string. When the plumb bob is allowed to hang freely, gravity pulls it downward, ensuring that the string is perfectly vertical. This characteristic makes it ideal for tasks such as aligning walls or establishing a true vertical line for construction purposes.

In contrast, a level uses a bubble in a vial to indicate horizontal alignment, which is not useful for establishing verticality. A square is primarily used for ensuring right angles in woodworking but does not provide vertical alignment. A chalk line is effective for marking long, straight lines on a surface, but it does not guarantee vertical alignment like a plumb bob does. Therefore, the plumb bob stands out as the appropriate tool for marking absolute vertical when suspended.