I have a dumb question regarding tippets: Is there any difference between IGFA tippets (rated by pound test) and regular (non-IGFA) tippets (rated by diameter; e.g., 5x)? I wouldn't think so, but I wanted to check. If there is no important difference, then shouldn't one be able to use an IGFA 6 lb tippet as a "4.5x" tippet, if the 4x tippet is 7 lb and the 5x is 5 lb test? (I take these numbers from the Orvis catalog describing fluorocarbon tippets.)

Orvis has a pretty good sale on their IGFA fluorocarbon, and I would like to buy some to use with my other non-IGFA Orvis fluorocarbon tippet if there is not a reason that this wouldn't work. If my reasoning is off, please help me to understand how I am mistaken.

Thanks