"IRWIN" PREMUIM LINE Length: 100" Ultra ThinIt features a premium thin chalk line and is to be used with our IRWIN Strait-Line Precision Chalk Reel. Ultra Thin is a braided premium line that is made of a polyester/nylon belnd. This line is only 0.5 mm in diameter and id ideal for exact marking and interior trim work. The thin line reduces chalk explosions making your mark cleaner.