What is fluorocarbon
Fluorocarbon is sold as the invisible line, and that is nearly true — but not because it is clear. Mono is clear too. What matters is how fluorocarbon bends light: almost exactly as water does. Everything else follows from that one property, including the fact that it usually goes on the end rather than replacing the main line.
What it is made of
Fluorocarbon line is made from the fluoropolymer PVDF. That is a different material from the nylon in mono and the polyethylene in braid — hence different behaviour, not merely a different price.
Why it is harder to see in water
The reason is not colour. Fluorocarbon is valued for a refractive index close to that of water, and that is why fish notice it less.
The distinction is fine but worth having. Clear line is still visible under water: light bends at the boundary between two media, and the boundary becomes apparent. When the material bends light almost as water does, there is barely a boundary — and so barely anything to see.
It sinks
A second property of the same material: fluorocarbon is denser and floats far less well than mono. Put plainly, it sinks.
That is not a fault but a working tool. Anglers reach for fluorocarbon exactly when a bait has to be held near the bottom without heavy lead. The line takes the rig down by itself, and there is no need to add weight that would coarsen the presentation.
Why leaders are made of it
Fluorocarbon is described as a leader material outright: strong for its specific gravity, clear, and with a refractive index close to water.
Set that against the fact that braided line is clearly visible in water and does not stretch at all, and the pairing becomes obvious. The main line supplies sensitivity and strength; the last metre of fluorocarbon supplies inconspicuousness at the bait. Each material does what it can do.
When it is not wanted
If the bait has to stay at the surface or in mid-water, the sinking works against you: fluorocarbon takes the rig down regardless of your plans. For float fishing monofilament is the logical choice, since it stays up.
Which is to say the choice between the three materials starts neither with price nor with the packet, but with a plain question: where exactly should the bait be, and what is stopping it getting there.
What we do not write here
Claims circulate about fluorocarbon that we do not repeat: how much more abrasion-resistant it is, whether sunlight harms it, how many times dearer it is. We found no verified source for these.
What is above is the behaviour of PVDF and what follows directly from it.