A chalk line is a tool used to create a straight and level reference line on a roof or other surface. The tool consists of a length of string or cord that has been coated with a chalky substance and is wound onto a reel or housing. To use the tool, the operator pulls out the length of string needed and snaps the string against the surface, leaving a visible line of chalk that marks the surface. Chalk lines are commonly used in roofing to create guidelines for the placement of shingles or other roofing materials, ensuring that they are installed in a straight and level manner. The chalk line can be adjusted as needed to accommodate changes in the roof pitch or to create angled lines as required.