The basic set of values was established two centuries ago: pawns are worth one; knights and bishops are worth three; rooks are worth five; the queen is worth nine. The king is trickier because, while it’s not so powerful in terms of mobility, it must be protected at all costs. The king cannot be captured and if it cannot escape inevitable capture, the game is over: checkmate. One trick is to assign the king a value of one million so the program knows not to put it in danger.
