Originally Posted by turbineblade
I guess "intended species" is a good thing to consider, but honestly I probably lean more toward "intended flies and conditions" now.
"Intended flies and condition" is generally in direct correlation with intended species. If I am out fishing for bluegill, I know that most of my flies are going to be pretty darn small so the 2 of 4wt are gonna generally be my choices. If I am throwing tiny dry flies for them in a small creek or pond, then the 2 will probably be my main choice. If I plan on throwing some "bigger" poppers, I'll use the 4.
If I am chasing smallies, I'll be using weighted clousers or some smaller deer hair poppers so the 6 is my usual rod for that. If conditions are extra windy or if I am throwing bigger poppers, I'll generally throw the 8 instead. Same goes for large mouth.
My point is that intended species and intended flies generally