Mibro's King Cord #18 mason's line is a twisted rope used to create straight lines and a level surface. It is needed when laying out posts, patios, footings and more. A mason's line is necessary when doing these projects because you will not have to pick up the level so many times. The bright colors provide excellent visibility and are ideal in any application where high visibility is desirable. The other great aspect of this twine is that it will not sag like ordinary twine tends to do.