Piano and Guitar Tuning

  I have tuned pianos by ear since I was in high-school and did it as a career for a number of years back in the 80's and 90's.  To be able to do that it was necessary to understand equal temperament, inharmonicity, octave stretching and a whole host of issues that challenge the most careful and obsessive music-geek.   I often see guitarists struggle with tuning who haven't had the benefit of this experience so I thought I would post a couple ideas that might help.
  Fortunately, guitarists generally don't have to deal with inharmonicity ( overtones that are out of tune with the string ) as that is a quirk of high tension strings found in pianos, unless the strings are old or defective.   Nor do they have to deal with octave stretching in the tuning process though that may be designed in to the scale.  But, equal temperament, on the other hand, is the backbone of guitar tuning.  The main feature of equal temperament is that all intervals, with the exception of octaves, are slightly out of tune.   Trying to tune by "ear" using randomly selected chords, major, seventh and the like is just an exercise in frustration. .  This will NEVER work.  The ear cannot discriminate pitch and subjective chord quality to the degree that is necessary to achieve equal temperament ( equally out of tune ) - it's just physically impossible for anyone's ear to do that.  Another thing that never works is trying to make use of "pitch memory" - the process where you hit a note on the piano then walk over and tune the first pitch based on your memory of that note.  Studies have been done to show that this is extremely unreliable.  Simultaneous sounding of the pitches or beat-rate comparison is the only way to get enough accuracy to establish a starting reference pitch.
      The net result of a good tuning is that all of the triads and like intervals are equally out of tune and this is virtually impossible if you're listening only to "chord quality" because in ET the best "chord quality" is not the goal.   We need something much more accurate that can achieve "equally out of tune chord quality".  Tuning adjacent strings to an interval of a 4th won't work either, because in ET 4ths are slightly out of tune to the wide side.  Furthermore, the major third interval between the G and B string can be a  real pitfall in subjective ear tuning because, in Equal Temperament,  this interval is extremely wide of perfect - it is impossible to judge aurally with any accuracy.  In aural piano tuning, thirds are always verified with reference tones and beat-rate comparisons, but that is a whole other post someday.
  Tuning success of course requires that the bridge, nut and frets are perfectly adjusted and that the scale design is good.  I once sat through a workshop for an hour by famed guitarist Johnny Smith that was entirely about tuning the guitar and realized the complexity of tuning and the many different approaches that a person could use.   The bottom line is that the best tuning ( perfectly consistent out of tune chords) is achieved using octaves or harmonics, comparing same pitches or octaves, never using the other smaller tempered intervals.  Or use a good electronic tuner - on a well adjusted guitar, the results are excellent.  The new generation of electronic devices for pianos are sensitive to 1/100 of a cent or more - the ear cannot detect that small amount, but it CAN detect the effect that such a small change has on the subjective quality of an interval.  Consider that when purchasing an electronic device for tuning.