They sit in the same place in the system, which is why beginners mix them up. The distinction is about function and replacement, not about position.

Leader

The leader is the whole length between the fly line and the fly, usually 7.5–12 ft. It is tapered: thick at the butt so it can transfer the energy of the fly line, thin at the point so the fly lands softly. A leader is a single unit — you buy it or build it, and you fish it until it gets too short.

Tippet

Tippet is the fine, level material you tie onto the end of the leader. It has two jobs: replace the length you lose to fly changes and break-offs, and let you change diameter without changing the whole leader. The number — 4X, 5X, 6X — describes the diameter, and there is a trade-off: thinner sinks faster and spooks fewer fish, but breaks more easily and turns over big flies badly.

A working rule

Situation	Rough choice
Big dry flies, wind, distance	3X–4X
Standard trout dries, 12–16	4X–5X
Small dries, clear water, spooky fish	5X–6X
Nymphs, streamers	3X–4X (or finer for tiny nymphs)

Add tippet when the leader has lost about 18 in / 45 cm — normally after a handful of fly changes. A loop-to-loop or double-surgeon’s join is enough; there is no need to rebuild a tapered leader by hand.

The reason the taper matters at all comes back to the cast: energy has to travel from the thick, heavy fly line into a thin, light leader without collapsing the loop. That transfer is described in the FFI Fly Casting Definitions under leader and turnover.