====== Chess - Programming - Polyglot Book Format - Weight ======

**Weight** is a measure for the quality of the move.

  * The probability that a move is selected is its weight divided by the sum of the weights of all the moves in the given position.

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**NOTE:** This is sometimes referred to as **count**.

The Polyglot book generator sets it to 2*(wins)+(draws), globally scaled to fit into 16 bits.

  * A move with a weight of zero is usually deleted from the book.
  * This is just a convention and book authors are free to set this field according to their taste.

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===== Zero Weights =====

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**WARNING:**  Older Polyglot versions are defective as to the use of zero weights, (meaning the move should never be played), and crash when they encounter them.

This because of a misplaced internal consistency check, which really only made sense during book building (which deletes all entries with zero weight from the book).

Yet hand-tuning of a book often leads to the desire to completely disable a move, without deleting it.

Some software approximates an entry with zero weight by giving it very low probability.

  * This is for instance what Scid does, but it means the move will still occasionally be played.
  * With XBoard you can edit Polyglot books to set the weight to exactly zero, as well as delete or add moves.

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