Glossary
Tippet
The thin, level line tied to the end of the leader and then to the fly, sized by an X rating.
Tippet is the thin, level section of line added to the end of a leader, with the fly tied directly to it. Because it is the last and thinnest piece of line before the fly, it is the part most likely to break on a strong fish and the part replaced most often as flies are changed or lost.
Tippet is sized with an X rating, where a higher number means a thinner diameter, so 6X is finer than 4X even though the numbers seem to run backward. Thinner tippet is less visible to fish and lets small flies move more naturally, but it also breaks more easily, so matching tippet size to fly size and fish strength is a core skill.
A common mistake is leaving the same tippet on for an entire trip regardless of what is being fished, when in fact most anglers keep several spools on hand and adjust as fly size or water clarity changes. Wind knots, nicks from a fish's teeth or a hook point, and UV-weakened line are frequent reasons a good fish is lost, so checking tippet by feel before it matters is worth the habit.