Resources
The official website of the original CQL software developed by Gady Costeff and Lewis Stiller. Contains great documentation and many examples, focusing primarily on using CQL with chess study databases.
Extensive information related to all aspects of regular expressions including tutorials, reference information, and examples.
Databases
- HHdbVII Endgame Study Database (Commercial)
The database that served as the original inspiration for CQL, provided in standard PGN format. The Harold van der Heijden endgame study database (HHdbVII) is the most comprehensive collection of endgame studies available. The meticulously curated studies contain a wealth of information presented in a standard format including study composers, publication information, details of known cooks, corresponding composition tournament results, etc. The most recent version of the database (released in 2025, updated every 5 years) contains 103,157 studies consisting of 5,209,799 positions across all variations.
- HHdbVI Endgame Study Database (Commercial)
The predecessor of the current HHdbVII endgame study database (above). Released in 2020, this version of the Harold van der Heijden endgame study database contains 93,839 studies consisting of 4,527,028 positions.
- The Week in Chess (Free)
Released in weekly installments for over 25 years, contains many high-quality games from major chess events worldwide. Available in PGN and CBV formats.
- Lichess Game Databases (Free)
All of the rated games played on lichess.org since 2013, grouped by variant and month in PGN format, released under the Creative Commons CC0 license. Games include date and time, links to the games on lichess.org, player names and ratings, ECO, Opening, TimeControl, and Termination tags. About 6% of games include Stockfish analysis annotations for each position. Games since April 2017 contain clock information comments for each position. With over 7 billion Standard rated games and over 140 million rated variant games, this is a great resource for various research purposes.
- FICS Game Databases (Free)
Access to over 200 million games played on the Free Internet Chess Server during the last 20 years in PGN format, grouped by year and month. Provides the ability to filter downloaded games by time control and rating. Includes several million variant games (crazyhouse, suicide, atomic, losers, etc.).
- Lichess Puzzles (Free)
Over 1.8 million original chess puzzles from games played on lichess.org. Puzzles are provided in CSV format and include starting FEN, rating information, and theme tags. The puzzles need to be converted to PGN format prior to consumption by CQLi.
Books and Periodicals
An electronic periodical with a wide range of original puzzles of various types by renowned author and Chess Master Jeff Coakley. Puzzles include the “Who’s the Goof”, “Triple Loyd”, and “Switcheroo” discussed in Generating and Solving Chess Problems as well as many others. Over 200 issues of of “Puzzling Side” are freely available in PDF format. The “Winning Chess Puzzles for Kids” books (volumes 1 and 2), also by Coakley, contain hundreds of similar puzzles and are a great resource. Finally, the Scholar’s Mate magazine (edited by Coakley) is a wonderful and engaging publication featuring puzzles developed by Coakley. PDF versions of the most recent 50 issues are freely available.