Fly Line Backing
Fly line backing is installed on a fly reel before your actual fly line, which is then attached to the backing and wound onto your reel. There are two main reasons for using fly line backing. Firstly, the spools of many fly reels had, and still do have, a fairly small diameter spindle. If you were to wind a fly line onto such a small spindle, the actual fly line can acquire some tight memory coiling, which can prevent the line from laying out nice and straight when it is cast. The addition of fly line backing increases the effective diameter, so when the fly line is attached and wound on, it is not so tightly coiled and is therefore less likely to suffer from memory coils.
Secondly, if you are fortunate enough to hook a large fish, it could be powerful enough to simply head off and take all of your 30yd fly line before you have chance to slow it down or stop it. The additional length of another 50-100yds or more of fly line backing gives you a fighting chance to play the fish properly until it is to hand! The modern designs of mid and larger Arbour Fly Reels have much larger diameter spindles and centres, which effectively reduces the coiling tendency and thus the need for lots of backing.
If you are fishing in locations such as streams, small rivers and smaller lakes where there are fewer snags, there will be no real need for a lot of high-tech backing - any good, rot-proof backing will do, since it is essentially there to pad out your spools. Lureflash Twisted Nylon Backing is excellent value and rot-proof, while Lureflash Braided Mono Backing is hollow and can also be used to make leader loops. Leeda Profil Braided Backing on 100m spools is woven and also presents a very good choice.
If you are fishing for larger, more aggressive species, and/or where there are likely to be more snags such as rocks and coral, a good amount of high quality, thin diameter, abrasive resistant fly line backing is the order of the day. Wychwood Truefly Backing is a good quality braided Dacron. Ron Thompson Energiser Braided Dacron Backing is also an excellent choice, as is Scierra Braided Dacron.