neck thru's, supposedly. It really depends on how well it's made. You can even get some bolt-ons that sustain logner than set necks.

The thing that makes it sustain is the transition of the string's vibrations into the body. With a solid neck joint, they can run through the frets, into the neck and body.

Also, one-piece necks are said to sustain more than 3 or 5-pieces, which kind of contradicts the neck-thru principal...

Aw, hell, I dunno.
Boss Sustainer pedal with foot knob!

no just kidding

Neck Thru for Both usually.

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Thruuuuuuus. And they look better, but all my guitars are bolt on (apart fdrom acoustic ) so....
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I know someone sith a charvel neck thru (its a neckthru in the sense of the body is the same piece of wood as the neck rather than having different pieces as the 'wings') and he says it sustains longer than anything he's played and he's played a hell of a lot, i think he's been playing about 5 or 6 years now, so probably NT
Sustain is longer if the body is of the guitar is more solidly constructed. Once a string is played, it would go on ringing forever if it lost no energy. But it loses energy in air resistance, bending of the metal in the string itself (into heat), but most of the energy is lost to vibrating the rest of the guitar. A more solidly built guitar will soak up less energy from the string and thus ring better. And because its arguable that through-neck is a more solid construction method than set-neck, which in turn is again more solid than bolt-on, you get superior sustain. Single piece necks with less joints will also be more solid than multi-piece necks.

Note that depending on build quality, you can in fact have a bolt-on multi-piece neck that has better sustain than a poorly made single-piece through-neck. So it's a combination of type and construction quality that leads to better or worse sustain.
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