weight is a measure for the quality of the move.

• 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 deleted from the book.
• This is just a convention and book authors are free to set this field according to their taste.

If random play is enabled in Polyglot then 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.

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.

Weight