Re: 12 String
I am afraid that I disagree with Jim (in the nicest possible way, of course).
Normal guitar tuning is to E and, in my opinion, if a 12er is built that will not stand the stress of tuning to standard tuning then it is designed incorrectly and is only fit for the rubbish skip.
I have had 12er's off ond on for the past 40 years and my current baby is a 70's, Italian made, Melody; my previous one was a new, made in China, Channel guitar and both have only known standard E tuning with no adverse affects.
Modern strings are available in lighter gauges than they were in my early playing days so I believe that there is no problem using standard tuning.
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