Parity is an error-checking procedure in which the number of 1s must always be the same—either even or odd—for each group of bits that is transmitted without error. In modem-to-modem communications, parity is often one of the parameters that must be agreed upon by sending parties and receiving parties before transmission can take place.

If a parity error occurs on the trailing byte of a stream, an extra byte will be added to the input buffer with a value of 126.