The following are the required horn signals listed in the operating rules of most North American railroads, along with their meanings. Signals are illustrated by an "o" for short sounds, and "–" for longer sounds. (Note that these signals and their indications are updated to reflect modern practice.) Those rules marked with an asterisk (*) must be sounded when or where applicable. Those signals without an asterisk convey information to employees; they must be used when voice communication is not available.