All Western Fly Rods come with a Line Weight Designation, like a 9 foot rod for a 5 Wt. Line, for example. T-rods do not have any information like that on the box. But it would be most helpful if they did. About as close as we can get (in English) is the designations of L L for level Line Rods, L T for rods designed to cast either Level or Tapered Lines. Generally speaking, L L Line rods are soft and Full Flexed, and have the lowest Penny Ratings and would include a very soft 4:6 or a 5:5 soft to stiff rod segment description. 6:4 rods could go either way but, 7:3 and 8:2 rods would be Tip-Flexed, Fast Action, Casting Rods.
As the line is cast, it is the energy stored in the rod that catapults the weight of the line out in the desired direction. The Western Fly Line Numbering System corresponds to the weight (in grains) of 30 feet of line less the taper, which for a 5 Wt. Line would be 140 Grains. Tenkara Level FC lines are also Numbered: Size - # 1.5; Dia in Inches - .0081; Lb Test - 6 Lb. Test - Approximately
# 2 .0093; 8 Lb.
# 2.5 .0102; 10 Lb.
# 3 .0112; 12 Lb.
# 3.5 .0122; 14 Lb.
# 4 .0130; 16 Lb.
# 4.5 .0138; 18 Lb. Test
But there is no, as far as I know, correlating weight standard or measurement for T-lines. It stands to reason that more line will weigh more than less line will, and thicker line will weigh more than thinner lines do. And the best rod casting characteristics will fall within the line weight range the rod was designed to cast most efficiently, even though we have no exact way of knowing what that range will be. But, we can determine a lot by feeling how the rod is throwing the line we are casting, and that’s where over and under lining the rod comes into play.
Say you are fishing a small, stair stepped, plunge pool stream, where not only are the casts short, but the length of the pools requires a very short line to be used or the fly ends up in the rocks and in the bushes. Your 3.3 m rod casts best with a # 3 line as long as the rod is long but, that’s too long for the pools you are fishing, and the rod casts poorly with the shorter lines of 5, 6, 7 and 8 feet because these shorter lines do not weigh enough to load your rod properly. The solution is to Over Line the rod by going to a bigger, heaver, line size. One that has enough weight to load the rod properly in the shorter line lengths this kind of water demands. Because the line is so short, the heavier line will not compromise holding your line up and off of the water because it does not weigh all that much in such short lengths.
At the other end of things, lets say you need very long lines to fool really spooky fish, requiring lines of 6 to 10 meters or more in length, and your 3.6 m long rod casts best with a # 3.5 line as long as the rod is long, + 4’ of tippet. Ten meters of a #3.5 line is going to over load the rod and you will not be able to get the distance or accuracy you need. But 10 meters of # 2.5 line will load your rod well enough to get the distance and cast accuracy needed if, (and this is a very BIG IF) your casting is good enough to use all the line’s potential. To get there just takes time and a lot of practice but, you can do it if you work at it. We all Can.