Fillet brazing is rarely used in bicycle frame construction today. It’s a beautiful, but labor-intensive construction method that requires considerable skill on the part of the frame builder. The “fillet” itself is a layer of brass melted into the frame joint at relatively low temperature, then filed smooth. Fillet brazed frames are easily recognized by the unique appearance of one tube flowing seamlessly into the other. In my opinion, the aesthetic qualities of fillet brazed frames rival those of lugged-steel. Current builders using at least some fillet brazing in their frame construction include Pereira, Nobilette, and MAP (among others).
Photo © Pereira Cycles (via Flickr)