6X is finer than 3X or 0X. In the standard X sequence, the number and the diameter run in opposite directions: a higher X number means a finer tippet, so the finest of these three is 6X, then 3X, then 0X.

• A. 0X
• B. 3X
• C. 6X
• D. All have the same diameter

Answer: C. That is worth saying out loud because the sequence is not a size scale in the usual sense. The cited chart lists 6X as finer than 3X, and the same rule carries through the whole sequence. If you remember only one thing, remember that the bigger number is the thinner line.

In practice the chart pairs tippet diameter with fly size, because the choice is a trade. A finer tippet presents a small fly better; a heavier one turns over a bigger fly more reliably. Pick the finest tippet that will still turn the fly you are fishing, rather than the finest one in the box.

Source

From the Orvis tippet diameter and fly-size chart; terms follow the FFI Fly Casting Definitions.