Tuning
the guitar to a piano
Of course this assumes that the piano is in tune.
Sometimes trying to tune to a piano that has not been tuned
in years can do more harm than good.

* One important thing to know about the guitar, especially
if you have played another instrument. Music for the guitar
sounds an octave lower than
where it is written. If you try and tune the guitar to the piano
thinking that they are exactly the same, you will break all
of your strings. You would be trying to tune the guitar an octave
higher than where it should be.
Middle C on the piano is written like this

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Middle C on the guitar is written like this

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Page 2, Tuning the rest of the strings
Page 4, pitch pipes 
Basic
guitar tuning jump zone