The term "plumb" refers to a line that is perfectly vertical--in technical terms, an imaginary line that runs straight to the center of the earth. In construction, a framing member or building panel installed plumb will transfer its supporting load directly downward, giving it maximum load-bearing capacity. Poor construction in which vertical elements are not installed exactly plumb will weaken the structure.
Determining plumb can be done with either a plumb bob or a carpenter's level.
A plumb bob is a very simple tool that features a small weight and a string. When the string is suspended at the top with the weight at the bottom, the weight will straighten the string to establish a perfectly plumb line that can be used for reference when installing building materials in a plumb orientation.
The standard has one or more plumb bubble vials that are arranged perpendicular to the line of the level. When the carpenter's level is held against the side of a vertical framing member, when the bubble in the plumb vial is perfectly centered within the vial, the framing member is perfectly plumb. To ensure that the framing member is plumb in both directions, the board must be checked on two adjoining faces.