210 denier is 210 denier and 140 denier is 140 denier. That is all you can be sure of.

Why the flyfishing world has to complicate something the garment industry has been using successfully forever is beyond me. Compare denier to denier, throw in breaking strength info if you can find it and compare threads that way, not by aught sizes.

Relating denier to aught sizes is a load, the same load that fly tying thread makers have been perpetuating since the fly tying industry began. How else do you explain different manufacturers calling the same aught size a different denier? They just won't give up the, "my 6/0 thread is thinner than your 6/0 thread" game and fly tyers fall for it all day long!

The sooner the hobby and industry gives up the aught designations the better off we'll all be.

Until that time all you can do is guess and pretend what you bought is 6/0 and 3/0 and if it works for you, size really doesn't matter!