When I started fly fishing in 1955, fly lines were designated with a lettering system, beginning at A and running through I. The letter designation specified a diameter, with “A” being the largest diameter and “I” the smallest. This brought line designations up to par with leader “X” designations, each “X” representing a diameter (for example, 3X = .008,” 7X = .004″). The diameter designations were OK in silk because every line was made of the same material and therefore weight designations were not important.

But, when plastic lines became available, the letter designation (a shorthand way of indicating diameter), was no longer sufficient. A size “D” floating line weighed considerably less than a size “D” fast sinking line. And since it is the weight of  the line that we cast and not its diameter, using diameter alone as the indicator of line size was no longer acceptable. Thus came the American Fishing Tackle Manufacturers Association (AFTMA) weight categories, designated with numbers 1 through 15. We’ll look at these in another post.

So, relatively early in my fly fishing career, manufacturers switched horses in the middle of the stream and began using the AFTMA numbering system. It was a great improvement, really,, and allowed some real refinements in tackle and tackle designations.

Here’s a chart that shows the old system of letter designation versus the new AFTMA system. The comparisons are for silk against a floating plastic line.

ALPHA   DIAMETER           DT          WF        AFTMA
#              IN INCHES        LINES    LINES         #

I                 .022                                                  1
H               .025                                                   2
G               .030                   IFI           IFG          3
F                .035                 HFH        HFG          4
E                .040                HEH         HEG          5
D                .045                HDH        HDG          6
C                .050                HCH        HCF           7
B                .055                GBG         GBF          8
A                .060                GAG         GAF          9
2A              .066                G2AG      G2AF        10
3A              .070                G3AG      G3AF        11
4A              .073                G4AG       G4AF       12