The concept is broad, and hard to define precisely. Definitions specific to subfields are common. For example, in information theory, a "signal" is a codified message, i.e., the sequence of states in a communications channel that encodes a message. In a "communications system", a "transmitter" encodes a "message" into a signal, which is carried to a "receiver" by the communications "channel". For example, the words "Mary had a little lamb" might be the message spoken into a telephone. The telephone transmitter converts the sounds into an electrical voltage signal. The signal is transmitted to the receiving telephone by wires; and at the receiver it is reconverted into sounds.