How a parity bit checks a message
Before sending a group of bits, a system counts its 1s and adds one extra bit, called the parity bit. In even-parity systems, the added bit makes the total number of 1s even; in odd-parity systems, it makes the total odd. For example, if a group contains three 1s and the system uses even parity, the parity bit is 1, bringing the count to four.
The receiver counts the 1s again, including the extra bit, and checks whether the result matches the agreed rule. If a transmitted bit changes from 0 to 1 or from 1 to 0, the count’s parity switches, so the receiver can flag that group as corrupted. The check is simple enough to add to many kinds of digital communication without complicated equipment.
A warning, not a repair
Parity can reveal that something went wrong, but it usually cannot identify which bit changed or restore the original message. That job requires other methods, such as retransmitting the data or using more elaborate error-correcting codes.
It also has a blind spot: if an even number of bits flips in the same group, the total can still satisfy the parity rule. The receiver may then miss the corruption. Parity is therefore a lightweight check against many common errors—not a guarantee that every message arrived intact.
