Fly Lines and Backing
Choosing suitable fly lines is arguably as important as choosing a rod, reel or fly, but most fly fishermen and women seem to have great difficulty with selecting an appropriate fly fishing line.
Choosing a fly fishing line is all about selecting the one that gives you best chance of getting your flies to where and at what depth your fish are. So, ask yourself where the fish are likely to be, then choosing the correct fly lines becomes a lot easier. Virtually all modern fly lines are made by extruding a molten coating of PVC onto a yarn core, much like the way a candle is molten around a wick. The shape and thickness of the fly line can be made to vary along its length to produce different profiles or types. There are essentially two types of fly lines - weight forward fly lines, and double taper fly lines. The weight fowards are a bit thicker/heavier at one end and thinner at the other end, tapering off at the front end. The idea is that if you have more of the weight nearer the front, when you cast, that extra weight will fly easier and faster, and will help to pull the lighter rear end after it.
Double taper fly lines are uniformly thicker throughout their length, except for about 2-3mm at each end where they gradually taper off to a thinner diameter. Double taper lines were the first design of floating PVC type lines, and were relatively easy to make. Improvements in technology has meant that weight forward lines and various other profiles have made DT lines pretty outdated. Floating versions in lighter AFTM weights are still used by some dry fly fishermen, mainly in river fly fishing, but have largely been superceded by weight forward lines.
The other factor to consider is choosing between the two densities - either floating, which means the the whole line floats on top of the water surface when cast out, or sinking, which means that the whole of the line will eventually sink if left alone. The sink rate can be changed by adding different types of heavy compounds to the plastic when the line is made. The sink rate of sinking fly lines is expressed in inches per second (IPS). An intermediate fly line is still a sinking line; it is simply a fly line that includes a medium weight additive so that the line will sink fairly slowly at about 1-2 IPS. A sinking fly line is heavier and usually regarded as having a sink rate of approximately 3-5 IPS. A fast sinking fly line would have a sink rate of approximately 6-8 IPS.
Finally, all fly lines are made to a general AFTM line weight, usually expressed as #7, #4, etc. This is simply an indication of how relatively heavy the first 30ft or so of the line weighs, which in turn will match the optimum weight needed to load a fly rod which is rated to match. Simply put, a #7 weight fly fishing line is the correct line weight to select if you are using it with a #7 weight rod.