another one...

[mikehanna]

Upon purchasing 1/4'' jacks, I asked myself this question...

(unfortunately, I didnt know the answer when I asked)

What is the difference between stereo and mono jacks and why?

[ethrbunny]

Stereo jacks / plugs have an additional tap on the end for the other channel. Often stereo plugs are used to turn an effect on / off depending on whether a cable is inserted by completing the circuit between the two tip channels (when using a mono plug).

[Samuel]

The difference is the number of connectors made available to you. In mono, there are two connectors - the tip, and the sleeve. This is all that is strictly necessary for transmitting a guitar signal - tip for the signal, sleeve for ground. The stereo jack has three connectors, tip, ring, and sleeve. This allows the cable to communicate stereo audio (right, left, and ground).

Since the sleeve is always used for ground, a stereo jack on the input (or output for some pedals, see ZVex...) allows for a bit of a trick. The power's ground connection is fed into the ring connector, and the ground for the circuit board is connected to the sleeve. That way, when a mono plug is inserted into the jack, ring and sleeve are connected together (since the mono plug has no ring) and power ground is at that point connected through the plug into the board, allowing power to flow to the circuit. That way the circuit cannot be accidentally turned on to drain the battery when it's just sitting in your bag...

[mikehanna]

is there ever a time/place when installing one type of jack can cause the circuit to fail?

[Samuel]

Well the stereo jack trick will cause the circuit to not function if a) it's not wired properly, or b) a stereo plug is inserted into the jack instead of a mono.