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A string line is one of the oldest and most basic of hand tools, and it is commonly used in masonry and carpentry to help the user create a straight line between two reference points. The shortest distance between two points is a straight line, and a string allows the user to mark the shortest distance, which helps to avoid sags or tilts in a structure. A tool that is hundreds of years old, a string line is typically either used in conjunction with a plumb bob, commonly referred to as a plumb line, or is chalked.
Both a string line and plumb bob are useful for marking straight vertical lines. Used with a plumb bob, which is a weighted object with a pointed tip attached to the end of the string, the user must mark reference points along the line between the starting point and the end point. This enables the mason or carpenter to maintain a straight line as he works as well as gives him other points to work off of. A string level can also be used to create a straight, level horizontal line.
A chalk line is another version of this tool. A small box holds a length of retractable string, which, when wound up inside, is coated in powdered chalk. There is a hook at the end of the line for attaching it to one point and when the line is pulled tight between the two points being marked, the user plucks or snaps the string. The chalk covered line then transfers a visible line to the work surface. This is a useful tool for marking horizontal lines once the start and end points have been determined.
A string line is also sometimes used in surveying, but in many cases, laser equipment has replaced it for this purpose. The use of a plumb line or a chalk line is often based on personal preference as well as the task being accomplished.
One time I was sent on a temporary work assignment with a roofing company, and I had never done any kind of roofing work before then. I was mostly asked to haul materials from site to site. One of the regular roofers needed help making chalklines, and he asked me to hold one end of a string while he stretched it out to a row of shingles. When the string was tight, I was supposed to pull up on it and let it hit the roof. The chalk would create a straight line he could follow later when he started shingling that part of the roof.
I was surprised to see how such a low tech tool like a string line could work better than a high tech laser leveler.