In data transmission and telecommunication, bit stuffing (also known—uncommonly—as positive justification) is the insertion of non-information bits into data. Stuffed bits should not be confused with overhead bits.

<protocol> A protocol which guarantees the receiver of synchronous data can recover the sender's clock. When the data stream sent contains a large number of adjacent bits which cause no transition of the signal, the receiver cannot adjust its clock to maintain proper synchronised reception. To eliminate the possibility of such a pathological case, when a preset number of transitionless bits have been transmitted, a bit which does cause a transition is "stuffed" (transmitted) by the sender. The receiver follows the same protocol and removes the stuffed bit after the specified number of transitionless bits, but can use the stuffed bit to recover the sender's clock. The advantage of bit stuffing is that only a bit (not a byte) is inserted in the data stream, and that only when the content of the data stream fails to provide a timing signal to the receiver. Thus very nearly 100% of the bits transported are useful data. In contrast, asynchronous transmission of data "throws away" a start bit and one or more stop bits for each data byte sent. (1996-04-23)

The bit is an item of a horse's tack. It usually refers to the assembly of components that contacts and controls the horse's mouth, and includes the shanks, rings, cheekpads and mullen, all described here below, but it also sometimes simply refers to the mullen, the piece that fits inside the horse's mouth.