 Extended Hamming code

Adding an overall parity bit to the Hamming code of length seven gives an eight-bit linear code with minimum Hamming distance four: every resulting weight is even, every original nonzero weight is at least three, and a weight-three word extends to weight four. Thus it detects up to three errors when testing codeword membership and corrects one error. Joint single-error correction and double-error detection uses the overall parity together with the seven-bit syndrome.