The thick filter slows everything down.
Chemex paper is much thicker than a V60's, so water drains slowly and holds back oils — that's why its cup tastes so clean, and why it takes a coarser grind.
What it is
Chemex filters are much thicker than most paper filters.
Why it matters
They slow the flow and hold back oils and fine particles, giving an exceptionally clean, light-bodied cup — and calling for a coarser grind than a V60 for the same brew time.
In practice
Rinse the paper with hot water first, with the thicker, triple-layered side against the spout. If the brew drags past the target, grind coarser.