Fly lines size numbers pertain to weight- the greater the number, the larger the line. Bigger lines are used with bigger flies and fish, as well as lighter lines are used for smaller flies and also fish. There are some handy graphes around to assist find out which line size they must be utilizing.
Wind and temperature level likewise play right into establishing appropriate line weight. Windy disorders can make casting a lot more difficult and also casters do not consistently agree whether a lighter or much heavier line is much more efficient in these problems.