Parity Block Check

A parity block check applies parity in two directions at once. Several bytes are arranged as the rows of a block. Each row gets its own parity bit, exactly as in section 2.2 B. Then an extra byte — the parity byte — is added at the bottom of the block, and each bit of that parity byte is the parity of one column. Because every bit in the block is now covered by both a row check and a column check, a single changed bit makes exactly one row and exactly one column fail, and the bit at their intersection is the one that changed. This is the only method in this topic that can locate an error rather than merely detect one.