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Chess Games Database Online

learning - Where can I find free training tools like detailed annotated chess openings, middle games and endgames in the PGN format? - Chess Stack Exchange

Opening Explorer : Premium Only ? - Chess Forums

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Does the win/draw/loss rate from the opening database also apply to lower rated games? - Chess Forums

Popularity of chess openings over time

what do the numbers for each variation in chess opening mean by? - Chess Forums

Spreadsheet Library - Chess: Openings Database

56 openings you need to know how to play. Gold collection

What is the single best chess opening for average players to master? - Quora