AB (game)

AB is a two-player number-guessing game that adapts the logical structure of the board game Mastermind to a purely numeric form. One player, the codemaker, privately chooses a four-digit number with no repeated digits, and the other, the codebreaker, tries to identify that number in as few guesses as possible. After each guess the codemaker responds with two counts rather than revealing any digit directly: the number of As, digits that are both correct and in the correct position, and the number of Bs, digits that are correct but placed in the wrong position. The codebreaker uses this feedback across successive guesses to narrow down the hidden number through deduction alone, without any element of chance once the secret number is fixed. Because it needs nothing beyond pen, paper and two willing players, AB has circulated informally wherever Mastermind-style deduction puzzles are known, and it has also been implemented as a simple web game.