A bit added into a bit stream during a process known as bit stuffing, in order to 1) ensure synchronization technique used in time division multiplexing (TDM) by avoiding long streams of 0 bits, 2) adjust for slight timing discrepancies between incoming bit streams when being multiplexed into faster links (e.g., multiplexing T1s into a T3 using an M13 multiplexer), and 3) prevent the appearance of the 0x7e flag character within an HDLC frame (4-zero suppression). See also bit, bit stream, bit stuffing, flag, HDLC, M13, multiplexer, synchronize, T1, T3, and TDM.