The quarter inch jack ("phone connector") is probably the oldest connector still in use today, and it's picked up a very wide range of applications. Which also means it's a huge mess in a live sound context, where a 1/4" jack could be any of:

• Unbalanced or balanced line level (~1V). Ex: a mixer to a powered speaker.

• Unbalanced instrument level (~200mV), high impedance. Ex: electric guitar.

• Unbalanced piezo level (~50mV), high impedance. Ex: contact pickup on a fiddle.

• Unbalanced speaker level (~30V). Ex: powered amplifier to passive speaker.

• Stereo line level (2x ~1V). Ex: output of keyboard.

• Stereo headphone level (2x ~3v). Ex: headphone jack.

• Send and return line level (~2x 1V). Ex: input to and output from an external compressor.

• Switch (non-audio). Ex: damper pedal on a keyboard, which would be normally open or normally closed.

• 1V per octave (~5V). Ex: older modular synth.

Here's a compatibility matrix with a bunch of these:

		If you plug _____
		balanced line level output	electric guitar output	powered amplifier output	stereo line level output	effects loop (mixer side)
Into ____	balanced line level input	✓	too low, bad timbre	boom!	left - right	no audio
	DI input	distortion w/o pad	✓	boom!	distortion w/o pad, right channel dropped	no audio
	speaker input	nearly inaudible	no audio	✓	nearly inaudible, right channel dropped	no audio
	stereo line level input	✓	too low, bad timbre, right channel no audio	boom!	✓	no audio
	effects loop (mixer side)	no audio, possible damage	no audio, possible damage	boom!	no audio, possible damage	no audio, possible damage

Note how few "✓" cells there are.

As much as possible, set things up to avoid 1/4" connectors. If you use Speakon to connect amplifier outputs to passive speaker inputs and XLR for balanced line level signals you'll avoid all the "boom!" outcomes, along with most of the ways of connecting an output to an output. You can't get away from them entirely, but ideally the only 1/4" cables on stage are short runs to DIs.