A string line is one of the oldest and most basic of hand tools and 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. A string line enables 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 a chalked string line.

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 line level can also be used to create a straight, level horizontal line.

A chalked string line, commonly referred to as just a chalk line, is another version of this handheld tool. A small box encases a length of retractable string, which when wound up inside is coated in powdered chalk. There is a hook at the end of the string line for attaching 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. A chalk line 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 the string line. The use of a plumb line or a chalk line is often based on personal preference as well as the task being accomplished. Many carpenters and masons may use a specific string line exclusively or may use different varieties interchangeably.