Flueliner
Fly lines with WF-profile (weight forward) are typically fly lines that have all the weight in front of a running line. The length of the tapers, and the tapers itself, varies a lot, according to the work, or how the line are to perform, the designer of the line wants the line to do.

The belly of the line can be devided into 3-4 sections; 1) a front section, typically the front taper, 2) a body section, 3) a rear section, typically the rear taper, and to the last, 4) a step, transition from the taper of the head and down to the running line.

Furthermore, the lines are classified in two different ways, 1) according to the aftma-standards, and 2) a pragmatic aggressive standard. Both standards take the weight of the 30 first feet into account.

Lineweight of the first 30 feet according to the aftma-standard
#5 - 150 grain / 9,75 gram
#6 - 160 grain / 10,4 gram
#7 - 185 grain / 12 gram
#8 - 210 grain / 13,65 gram
#9 - 240 grain / 15,6 gram
#10 - 280 grain / 18,2 gram

Lineweight of the first 30 feet according to the pragmatic standard
#5/6 - 200 grain / 13 gram
#6/7 - 240 grain / 15,6 gram
#7/8 - 275 grain / 17,875 gram
#9/10 - 350 grain / 19,5 gram
#11/12 - 350 grain / 22,75gram

As you can see, the line weight for the first 30 feet for the two standards differs a lot. As a flyfisherman, you can interpret this in a pragmatic way. Look at the aftma-classification as the lightest recommended line weight a rod in that line weight would handle for an experienced flyfisher, and, use the pragmatic scale as a guideline for the heaviest line weight to be used by an experienced flyfisherman for a rod in that particular rod classification.

Most important of all, is to acknowledge that the weight-window for all fly rods are wide, meaning that one rod can handle a variety of weights within its weight-window, and typically that window allows for lines lighter and heavier than the rod is typically rated for.

Therefore, if you have a 6-weight rod, most lines between 160 grain/10,4 gram and up 240 grain/15,6 gram, will work well, as long as the flyfisherman is capable of adjusting the technique.

However, I would typiccally recommend this a a rule of thumb

1) For fishing for trout in rivers with small dry flies from size 12 and smaller, with shorter casts from 30-70 feet, under conditions with no wind and where a quite presentation is very important, and wehre you are using tippets for 7-5x, choose lines that matches your rod ratings with lines according to the aftma-classification, or go one line weight up!

2) For fishing with larger flies, heavier nymphs, larger dries, with tippets of 4x and stronger, maybe you have to cast 60 feet plus, and even in windy conditions, choose a line that is one or two line weights higher to your rod according to the aftma-standards, or just go for a pragmatic, aggressive fly line classification that matches the line weight recommendation for your rod.

Cortland Compact Sink Type 3
Cortland Compact Sink Type 3
Cortland Compact Sink Type 6
Cortland Compact Sink Type 6
Midgar Balance Taper, Ghost Tip 13
Midgar Balance Taper, Ghost Tip 13

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