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(7 Playing Strings Plus 12 Sympathetic Strings)
The Sympathetic Strings
 
 
McLaughlin's guitar in Shakti also had seven sympathic strings.  I became curious to try that, too, asked around, and was turned on to the tuning mechanism of the Portuguese guitar, which facilitates twelve strings:

I wood-glued half-inch-thick blocks of wood to the inside of the face of the guitar, behind the tuning mechanism and the fastening piece for the other end of the strings, so that the fastening screws would have something to grip into and the neighboring guitar face would be supported.  Initially the sympathetic strings weren't going to be low enough to pass under the playing strings, but I fashioned the brass bar just below the tuning mechanism, under which the strings run.

It ends up that the sympathetic strings don't vibrate sympathetically enough to hear.  They also fade pretty quickly, because they're pretty short.  I'm sure they'd vibrate more sympathetically if they were longer, had lower pitches, and were parallel with the playing strings.  They do sound great when strummed and tuned up well, but they're a little difficult to tune up.
 

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