OPENING CHOICES AT A GLANCE
Let's open up a chess tree and see what it contains. As I said, pretty
much every chess playing program has some sort of opening book; the
better programs let you see the tree's contents. One reason why chess
programs have opening trees is so that you, the user, will see a variety
of openings when you play against a chess engine; if left on their own,
chess engines will tend to play the same opening moves over and over.
But another reason chess playing programs have opening books is for
you, the user, to use them to help you study and research opening variations.
Here's Chess King's opening tree, showing the options for
White's first move:
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chessking.com