We often speak of the band within which information is communicated. This comes from radio terminology where all information-carrying signals are recognized to occupy a range of frequencies, hence a band of frequencies, or simply, a band. A wider band allows more/faster information flow. So lots of bandwidth is a good thing.

Even with lots of bandwidth, there are some signals that are best sent some other way. (This is really a coding issue, but the terminology holds.) The phone company used to send switching control signals as tones in the same band (over the same wires) as the customer's voice. Some customers discovered that they could generate the tones themselves and make free phone calls. This gave in-band signaling (InBandSignal''''''ing) a bad name.

The stream of keystrokes on a computer keyboard constitutes a band of sorts. Besides the alphabet and numbers and punctuation we have things like alt-tab to go between windows and ctrl-alt-del to start over. Wiki made a big mistake by assuming tab would remain in-band all the way to the wiki script. On some machines it does, on other machines it doesn't, causing all manner of suffering.

Flipping off and on the power switch is an out-of-band way to get the attention of most computers. On my laptop I find I must first remove the battery to make this signal effective.

There are dozens, maybe hundreds, of in-band ways to say TheEnd to various computer programs. With the advent of window systems, program designers now have many signals available to them outside of the keystroke-band. Just pointing the mouse at the window you'd like to talk to is an excellent way to invoke switching. Early window systems merged mouse motion signals with the keyboard stream making them once again in-band and subject to various abuse. Whenever the cursor stops following your mouse you can be sure you are suffering from some legacy of this design.

''Of course Unixy systems have the well-documented "kill" signals that can be sent to a process to make it (among other things) die.''

----
One could also apply the OutOfBand concept to interpersonal communication, I guess. -- OleAndersen, OpenAuthor
----
Is the referee's whistle OutOfBand? When followed by a yellow card, I suppose.

''It's out of band if we take the signal to be the interplay of the other players, the ground, and the ball.''

----
WindowsNt takes CtrlAltDelete out of band...

----

See OutOfBandChannel BackRoomDecision