The construction of fly lines took a huge leap forward when shortly after the second world war, manufacturers began to build lines with a level braided nylon core coated with plastic. The taper in the line is built into the coating, not into the core. With this technique, lines could be made that floated better than silk and others that sank better than silk. Floating lines were made by adding microspheres of glass filled with air to the plastic coating. To make the lines sink, the manufacturers replaced the air filled microspheres with powdered lead. For the sinking lines, manufacturers also started using a braided Dacron core rather than nylon.

Since the early days of building plastic coated lines, there have been improvements in the microspheres, other materials besides lead are widely used in sinking lines. tapers have improved, coating additives have changed, and so on, but the basic structure is still the same.

When manufacturers switched to the new methodology of making lines, they very quickly discovered that the old letter designation system (see the last post) was no longer adequate. The reason is very simple. A floating fly line of “D” diameter (.045″) was much lighter than a silk line of the same diameter, while a sinking line of the same size was much heavier than the silk line. And, since a fly rod is desigtned to cast a specific line weight, a “D” floating line was too light for a rod designed for “D” silk, and the “D” sinking line was way to heavy.

Manufacturers quickly gave up the diameter designation in favor of a strictly weight-based system. The plastic-coated line designation is now a number category, 1—15. Each number indicates a specific line weight. So, a number 5 floating line weighs exactly the same as a number 5 sinking line, even though the floating line is significantly larger in diameter. Furthermore, every 5 weight line, regardless of the taper style is the same weight. All these lines weigh the same: WF5F, WF5S, WF5I, WF5F/S, L5F, L5S, L5I, L5F/S, DT5F, DT5S, DT5I, DT5 F/S, SH5F, SH5S, SH5I, etc. A 5 is a 5 is a 5. A 5 by any other name is still a 5. That’s the beauty of the system. If one owns a 5-weight rod, that it will properly handle any 5-weight line, regardless of taper or density.

Line weight is the actual weight of the first 30 feet of line in grains (7000 grains per pound; 437 1/2 grains per ounce). Manufacturers often add a short tip (6 -12 inches) of level line at the very front of the taper on WF and DT lines. The weight of this section is excluded. Here are the categories:

Weight    Actual Line   Allowable
Class      Weight (gr.)   Variation (gr. +)
1                  60                         6
2                  80                         6
3                  100                       6
4                  120                       6
5                  140                       6
6                  160                       8
7                  185                       8
8                  210                       8
9                  240                      10
10                280                      10
11                330                       12
12                380                       12
13                450                       15
14                500                       20
15                560                       20