Sinking fly lines are heavy lines with admixtures of lead and tungsten powders into their cover, and are therefore used when fishing with streamers in deep and fast flowing rivers or when still water fly fishing in cases where we want to get our flies as fast as possible to the deep.
Sinking fly lines are marked with the Roman numeral I - VI (even more), which indicates the speed of sinking. I = about 5-6 cm / sec .; II = 7-8 cm / sec .; III = 9-10 cm / sec, IV = 11-12 cm / sec, etc.