In correspondence chess, players have always used study materials. Since the advent of computers, much has changed.
1. More people play turn-based chess than ever played traditional correspondence chess, and that includes growing percentages of players who play no other form of chess, often are beginners, and often have limited or no exposure to the traditions and culture of slow chess.
2. Engines are sometimes encouraged, sometimes banned, sometimes silently tolerated.
3. Databases are considered the equivalent of printed books, but not consistently.
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