Unlike other angling methods where favorable areas are the target of the angler's casts, the fly rodder must actually see the individual fish, determine its speed and direction, stealthily maneuver himself until the target is in range and then place an accurate but quiet cast. Fish like bonefish and reds, whose feeding habits take them from the security of deeper water onto shallow flats, know they're vulnerable, and rely upon keen instincts to shield them from danger. Once hooked these fish will bolt to deep water with astonishing speed, and like a 15 round fight that's over in the first round, they're fully capable of taking your entire tippet, line, and backing with them when they go. A number six rod, therefore, is manufactured to throw a 6 weight line and so on. The heavier the line weight, usually up to 15 weight, the stiffer the rod spine and consequently the more effort required to throw it.
Rods also vary in action from fast to slow.
Fast rods rely upon the tip while slower ones utilize a broader portion of the rod's length to propel the line and fly. My arsenal contains rods ranging in weights from seven to ten, and although each has its place in my heart, I find that I use my eight weight most of the time. The venerable eight is heavy enough to throw moderately large flies into a wind, can handle tarpon up to 30 pounds, and yet is benign enough to cast all day. Furthermore, an eight weight is the perfect size for bonefish, reds, and trout, and during windy days can be rigged with seven weight line, to facilitate throwing small flies into a brisk headwind. The smaller diameter line has less wind surface and seemingly pierces the wind better.