The phrase refers to a popular community-curated dataset where millions of Lichess puzzles are organized into zip files based on their Encyclopedia of Chess Openings (ECO) codes . What the Zip Files Contain
These zip files are typically PGN (Portable Game Notation) collections structured so you can practice tactics specifically related to certain openings: Lichess Puzzles By Eco zip
: Each puzzle entry includes the starting FEN, the move sequence to solve it, the puzzle's rating, and "Themes" tags (e.g., fork, pin, mate in 2). The phrase refers to a popular community-curated dataset
While the official Lichess database provides puzzles in a single large CSV or BZ2 file, the "By ECO" organization was a specific project shared by community members: download all puzzles - Lichess.org the move sequence to solve it