The line and leader are tapered to make your presentation better. Fly rods
and fly lines are rated with a number system that ranges from 1 through 12,
with 1 the lightest and thinnest and 12 the heaviest and thickest.
Lighter lines are more delicate. Heavier lines are needed to throw bigger
flies and to cast farther, especially when you have windy conditions.
Luckily, fly rods made for light lines are more flexible to protect light
leaders, and heavier fly rods have enough power to make long casts and
enough reserve power to fight big fish.
For most trout and panfish, size 4 or 5 rod and line is about right. Rods
lighter than a 4 weight are used for small fish or very delicate
presentations and are really considered specialty rods.
For a good all-round rod for trout and smaller bass, a size 6 is often
used. For bigger bass, smaller pike, steelhead, salmon, and smaller
saltwater species, a size 8 is the most popular size.
For very large flies and very heavy fish, size 10, 11 or even 12 might be
Don't forget a fly rod should always be matched to the correct line size,
otherwise it won't perform at its best.