A line marking assembly for applying a marking a straight line with a marking powder onto a surface is disclosed. The line marking assembly comprises two chambers configured to mount one on each side of a level. Each chamber contains a cord wound on a spool, marking powder and a tensioning assembly that exerts a constant and continuous pull on the cord. The cords from each of the chambers may be combined with a coupler. Moving the coupler back and forth between the chambers coats the cords with marking powder. Placing the assembly onto a surface and snapping the cord marks a straight line on the surface.