The American Fishing Tackle Manufacturers Association (AFTMA) is universally recognized to set the standard for Fly rod and Line specification. To accurately compare and match rod and line, exactly 30 feet of float line is weighed in “grains” which will fully “load” the rod to capacity for casting purposes at that distance. This is the maximized point of performance for storing and releasing energy as the rod swings and bends through the force created by line mass. 3, 4, 5, & 6 weight rods swing lines with corresponding numbers that weigh 100, 120, 140 & 160 grains respectively. You can see there are 20 grain increments as the strength of the rod increases. In general most lines should fall within a small variance of the weight ranges and most rods will fall within a stiffness range that suits the acceptable mass as well. At 30 feet, the intended average casting range, a five weight line loads the five weight rod perfectly. It stands to reason that if you have a five weight rod and you are going to cast only 20 feet a six weight line might actually load the rod more adequately and if you are going to cast 50 feet, a four weight line might actually load the rod best because of the total mass of that length of line. This condition has been the root of much confusion and debate. That being said, overall, the five weight line will function just fine over the entire spectrum of the casting range clear out to its entirety. Other considerations would be line tapers such as the “Weight Forward” line which is heavier in the first ten feet than it is in the second and third ten foot sections. This may promote better rod flex at shorter distances yet still meet the 30 foot weight requirement. Generally speaking the rod weight specification is influenced by the diameter and/or the type of material in which its designed. Rod length and taper design also affect its capacity. For a more technical perspective see www.common-cents.info. In general heavier weight rods cast further distances and handle heavier lures and more powerful fish. Montana Rodsmiths uses the “common cents” methodology to classify rod and line.