The smallest particles produced when you grind coffee — typically below 100 microns. Every grinder produces fines, even the best ones. The question isn't whether they exist; it's how many you get and how evenly they're distributed across the bed.
Fines extract very fast because their surface area per unit mass is huge. In moderation they add body and a sweet bottom-end to the cup. In excess they over-extract while the rest of the bed under-extracts, give you bitterness or astringency, and physically clog the filter so the brew stalls.
Why grinders matter so much
Two grinders set to the same average particle size can produce wildly different fines counts. Grind distribution — how tight the spread is around that average — is what separates a good grinder from a great one. Conical burrs typically run more bimodal (a peak of fines plus a peak of medium particles); flat burrs tend to be tighter and more unimodal.
Recognizing a fines problem
The clearest signal is a slow drawdown that gets worse as the brew progresses. The bed plugs. Pull the spent puck out and look at the bottom of the filter — heavy fines accumulation looks like wet brown plaster smeared against the paper. Coarsen one click. If the issue persists across many recipes and beans, the grinder itself is the problem.
