Let me cite several examples of when you might want to use various line weights on the same rod for different fishing conditions you may encounter.

First, if you fish a swift, tumbling mountain brook, you can use a rather short leader with a dry fly. A leader of 7-1/2 feet in length would probably do the best job. But if you fish for trout with the same outfit and dry fly on a calm spring creek, beaver pond or quiet lake, that short leader could prevent you from catching many fish. While many fishermen automatically know that on calmer water they have to use longer leaders, many of them don?t really probe any deeper into "why" they need a longer leader.

It isn't the leader's length that's so important. In calm water, what frightens the trout is the line falling to the surface. The longer the leader, the farther away from the fly is the splashdown of the line.

But with a longer leader, the more difficult it is to cast and there is a reduction in accuracy. Thus, a 9-foot leader is more accurate and easier to turn over than a 15-footer. Considering this, plus the fact that the splashdown of the line is what is frightening the trout, there is a simple solution. Use a fly line one size lighter than the rod manufacturer recommends.
Jim Green, who has designed fly rods for years and is a superb angler, almost always used a line one size lighter when fishing dry flies where the trout were spooky or the water was calm. I tried it and have routinely followed his advice. So, for example, if you are using a six weight rod, you can drop down to a five weight line with no problem. In fact, in very delicate fishing conditions, I often drop down two sizes in line weights. There is a reason.

Before you start putting line on your reel, find out how much backing you'll need by reading your reel instructions. Some fly lines are longer than others and some will reduce the capacity of reel because of the diameter of the belly. As a general rule of thumb, less backing is used on reels that will have Double Taper (DT) fly lines than Weight Forward (WF) Lines. This is because a Double Taper has a more even diameter in the belly than a WF line. If you purchase a reel at Flymart, they will load the reel up with backing and make these connections for you. If you are rigging your own outfit, this page will give you every thing you need to know to get going.