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The codebreaker tries to guess the pattern, in both order and color, within eight to twelve turns. Each guess is made by placing a row of code pegs on the decoding board. Once placed, the codemaker (in this case Gustave) provides feedback by placing from zero to four key pegs in the small holes of the row with the guess. A black key peg is placed for each code peg from the guess which is correct in both color and position. A white key peg indicates the existence of a correct color code peg placed in the wrong position.
If there are duplicate colors in the guess, they cannot all be awarded a key peg unless they correspond to the same number of duplicate colors in the hidden code. For example, if the hidden code is red-red-blue-blue and the player guesses red-red-red-blue, the codemaker will award two black key pegs for the two correct reds, nothing for the third red as there is not a third red in the code, and a black key peg for the blue. No indication is given of the fact that the code also includes a second blue.
source: Wikipedia