chalk line

Wiktionary

1. n. A line made on a surface by snapping a taut string or cord dusted with colored chalk.

GNU Webster's 1913

1. n. a cord rubbed with chalk, used for making straight lines on boards or other material, as a guide in cutting or in arranging work.
2. n. a string impregnated with powdered chalk, used in construction to mark a straight line on a surface, such as a wall. The string is held taut on the surface between the two end points of the line to be made, the center of the line is drawn away from the surface, and the string is released and snapped against the surface, leaving a line of chalk in the desired location.
3. n. the line of chalk on a surface left by a chalk line{1}.

WordNet 3.0

1. n. a chalked string used in the building trades to make a straight line on a vertical surface