What is a plumb line? How to find a perfect vertical line?

A plumb line is a piece of string with a weight. or plumb bob, on the end. To use it, suspend the string from a nail, hook, or other object so that the bob hangs freely. The string is perfectly vertical.

Mark the spot beneath the paint of the bob, or align a structural piece, such as a wall stud or a fence post, with the string,

Sometimes, as when wallpapering, installing tiles, or erecting shelves, it is useful to mark vertical lines on a wall. To do this, rub the string with colored chalk: then, after you have found and marked the plumb, stretch the line taut and snap it. Self-chalking lines are available, in which 50 or 100 feet of string are contained in a reel that doubles as a plumb bob.