To enable the receiving end of a data or telegraph link to interpret the received code words in a meaningful way, the receiver must first be synchronized to the incoming bit stream by finding the bit level transitions, and next achieve code word phase. Basically the receiver will search for a certain bit pattern in the bit stream and when found transmitter and receiver are synchronized.

Before the widespread use of electronic circuits, all telegraph devices were of electro-mechanical nature and therefore prone to mechanical wear and tear. This in turn necessitated comparatively large tolerances and made stable synchronization over even short periods difficult. To overcome this serious problem, the ITA-2 alphabet adopted what is known as start-stop or asynchronous operation, which achieves synchronism for each codeword.

In start-stop systems a codeword is wrapped into an "envelope" consisting of a leading start bit (logical '0') and one or more trailing stop bits (logical '1') - for ITA-2 the code words are 1 + 5 + 1.5 = 7.5 bits long. Bit synchronization is then achieved by detection of the start element. The stop element(s) serve(s) the purpose of telling the receiver to reset its detection mechanisms and wait for the next start bit. To ensure proper operation of the mechanical devices the stop bit was extended to have 1.5 times the length of a data bit, which accounts for the term "non-integral" earlier in this section.

In synchronous systems there is continuous synchronization between the sending and receiving devices either by special non-printing control characters being inserted into the messages at regular intervals, or the code words themselves being constructed to facilitate synchronism. To maintain synchronism, special idle or sync characters are transmitted when no traffic is transmitted. In contrast to start-stop systems, only elements having a duration of an integer multiple of the duration of the minimum signal element are used – isochronous sequence.

For burst mode or packet transmissions a leading preamble of either a sequence of alternating zeros and ones and a repeated fixed pattern – in satellite transmission called a Unique Word - is often used for synchronization purposes. A very common way of combined synchronization and packet delimiting is to pre-cede and terminate the data packet with so called flags, i.e., unique bit patterns, not being a part of the data proper. Flags are characteristics of HDLC (High-level Data Link Control), which is used in amateur radio packet communication and in AIS.