What makes a guitar sustain?

[Rich T Fingers]

I don't believe that there has been a definitive technical discussion on this key subject in these pages. I've seen sustain noted as being created by the ability of the strings to "ring on" without impedance from the body of the guitar which is why the Les Paul has a heavy body. However, the most sustaining guitar I have is a PRS CU24 trem with a WT (thin) neck so in theory it shouldn't sustain much. Moreover though, the next most sustaining guitar I have is a PRS Singlecut stoptail with a Brazilian neck like a tree trunk! Two totally different guitars that both sustain for days.
Clearly, individual guitars will embody factors that produce sustain outside of the ground rules.

But generally speaking, what are the ground rules : what in your opinion are the key factors that contribute to sustain?