In electrical engineering, a line is, more generally, any circuit (or loop) of an electrical system. This electric circuit loop (or electrical network), consists of electrical elements (or components) connected directly by conductor terminals to other devices in series. Electrical engineering is an engineering discipline that deals with the study and application of electricity and electromagnetism. ... There are many kinds of circuit An electric circuit interconnects electrical elements or electronic elements together usually to perform some useful function. ... The article on electrical energy is located elsewhere. ...

Specifically, consumer electronic devices concerned with audio often have a connector labeled "line in" and/or one labeled "line out". "Line out" provides an audio signal, "line in" expects one.

The signal out of "line out" remains at a constant level, irregardless of the current setting of the volumne control. So you can connect recording equipment to "line out" and record the signal, without having to listen to it through the device's speaker.

Acording to the ??? standard, the impedance of the "line out" is ??? Ohm, and the voltage levels are at ??? Millivolt. This impedance level is higher than the usual 8 Ohm of a speaker, so a speaker connected to "line out" essentially short-circuits this connector. Even if the impedances would match, the power supplied through "line out" is not enough to drive a speaker, anyway.

On the other hand, the final power amplifier stage of a typical audio device often introduces impurities. But "line out" is derived from some point before that final amplification takes place. So "line out" signals tend to be of higher quality than those out of a speaker (or headphone) connector.

"Line in" expects the kind of signal "line out" provides. So you can typically connect the "line out" connector of one device with the "line in" of another. However, if you do this with a straight cable, and both devices are AC powered, you may run into ground - loop - hum.