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Mastermind
Original rulesetMastermind

Classic 6-Colour

Hidden 4-peg code, 6 colours, 10 guesses. Black pegs for correct colour-and-position, white for correct colour-wrong-position.

Medium 5-15 min logic · deduction · code-breaking · classic

The deduction board game

Classic Mastermind: the codemaker picks a hidden sequence of 4 pegs, each chosen from 6 colours (repeats allowed). The codebreaker has 10 attempts to crack it. After each guess, the codemaker responds with:

  • A black peg for each peg that is the right colour and in the right position.
  • A white peg for each peg that is the right colour but in the wrong position.

The order of black/white pegs in the response is intentionally arbitrary — you know how many of each, not which of your pegs earned them. That ambiguity is the puzzle's engine.

Why it's elegant

Mastermind is a near-perfect teaching puzzle for constraint propagation. Each guess-and-response pair prunes the space of possible codes; an optimal player using Knuth's algorithm can solve any 4-peg/6-colour code in 5 guesses or fewer, with an average of 4.476. Casual players typically need 6–8.

How to play

  1. The codemaker sets a hidden code of 4 pegs, each one of 6 colours (repeats allowed).
  2. The codebreaker submits guesses — 4 pegs each.
  3. After every guess, the codemaker responds with:
  • 1 black peg per peg that's the right colour AND the right position.
  • 1 white peg per peg that's the right colour but a wrong position.
  1. The order of black and white pegs is arbitrary — you don't learn which specific pegs earned which response.
  2. Crack the code within 10 guesses to win.
  3. Pegs can repeat in the code — guessing four-of-a-kind is legal.

Strategy tips

  • Use Knuth's first move: 1122 (or any two pairs). It maximises information from the first response.
  • Track every consistent code. After each response, eliminate every code that wouldn't give the same response to your guess — only the consistent set remains.
  • Guess to learn, not to win. Early guesses should be chosen to maximally split the remaining possibility space, even if you "know" they're wrong.
  • Don't reuse colours until you've tested them. If you've used all 6 colours in 2 guesses and gotten no whites, those colours aren't in the code.
  • White pegs are local information. A white peg narrows colour-set but not position; use it accordingly.

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Other Mastermind variants

ModernWord Mastermind

Same deduction engine, but the code is a 5-letter word. The Wordle-precursor that predates Wordle by 50 years.