A guitar's volume control effects tonal quality as much as any other mechanism. The musician is able to "bottle up" the sound produced through the electric guitar by lowering the volume output using the knob located at the heel of all electric guitars. This does more than simply quiet the guitar's sound; it shrinks the sonic range of tones coming from the pickups through the amplifier. Volume control is the easiest method of tonal control for the guitar player, as it involves the least instrument manipulation.
The majority of electric guitars include tone control knobs grouped with the volume knob. These are most commonly separated into treble and bass controls. Manipulation of these controls enables the guitar player to push the tonal spectrum of the electric guitar from the hollow, ghostly quality of pure bass (think Alice in Chains) to the cut and warmth of pure treble (Eric Clapton).
Electric guitars, like the Fender Stratocaster, have pickup selector switches which allow the player to cycle through activation of the guitar's multiple pickups. This has great tonal manipulation potential, as each pickup (three in this case) produces a unique tone, from a warm shimmering sound quality to the twang of a country & western star.
Effects pedals or "stomp boxes" allow a guitar player to incorporate a tonal quality not possible through a direct amplifier-to-guitar connection. These pedals shape tones by producing various effects. For example, wah-wah pedals manipulate treble, while a "fuzz face" stomp box produces distortion through the abrupt subtraction of bass. Effects pedals can almost always be turned on and off with a foot switch, enabling the player to quickly cycle through tonal qualities and pre-set effects.
Musical Instrument Digital Interface has risen to prominence in the guitar world with the development of USB connectivity and the digitizing of amplifier models and synthesizer effects. Musicians using guitars with MIDI capability have access to a wide array of tonal effects and qualities, emulating other instruments to the production of otherworldly tones that sound like they could not be produced by a stringed instrument. MIDI tonal controls have been used to great success by such bands as MGMT and Muse.
The amplifier is the last means of tone control for the musician. A quality guitar amplifier will have volume controls for each of its channels (clean and distorted) as well as bass, mid, treble, and reverb (echo) control knobs. Musicians are highly sensitive to tone quality (bordering on superstitious), meticulously working amplifier controls until the "perfect" tone quality is reached.