Many experts agree that these guitar knobs actually do nothing. However, at times, you can turn them and the resulting effects are apparent when the guitar is plugged into something and you can hear an amplified output (it can be through speaker or headphones). The VOL is the most common knob you can find on any guitar. It is attributed to adjusting the volume of the guitar signal. Here the tone knob is another commonly found item. It is used to adjust the frequency spread of the signal going to the amplifier system. You can also use it for adjusting frequencies on stereos, radio and other sound output devices.