fly lines

The nature of the line is critical in fly fishing, because the line is more massive than the fly, and it is actually the line which is cast.

In the 17th century the line was horsehair, but the fly was dropped on the water, not cast. Later fly lines were made of silk, and the sizes were identified by letters on a scale from A (0.060 inch in diameter) to I, decreasing in diameter by 0.005 inch with each step, so I was 0.020 inch in diameter.

The old system wasn't capable of describing all the ways the nylon and Dacron lines introduced after World War II differed. In particular, it didn't indicate density: nylon is less dense than silk, and Dacron denser. A new system was introduced in which letters indicated construction. “HCH”, for example, was a double taper line and “HCF” was a weight-forward line. Descriptions were sometimes added to the designation, such as “HCH sinking Dacron”.

In 1961, the American Fishing Tackle Manufacturers Assn. introduced a new system based on describing by a series of numbers the weight of the first 30 feet of the line, exclusive of any untapered tip on a tapered line.

No.	Grains	Permissable range in grains
1	60	54–66
2	80	74–86
3	100	94–106
4	120	114–126
5	140	134–146
6	160	152–168
7	185	177–193
8	210	202–218
9	240	230–250
10	280	270–290
11	330	318–342
12	380	368–392

Important characteristics were assigned code letters:

Code	Characteristic
L	level line, constant diameter
DT	double taper
WF	weight forward
ST	single taper
F	floating
S	sinking
I	intermediate

A line is designated by giving the symbol for the taper, followed by the number for the weight, followed by the symbol for its sinking/floating characteristic. So, for example, a DT-9-F line would be double taper, the first 30 feet measured from the beginning of the taper would weigh between 230 and 350 grains, and it would float.

drawing of fly line

Notice that this system does not take into account the diameter, finish, composition, or braid of the line.