Here at MayosMusic, in addition to offering great guitar, bass, ukulele, banjo and autoharp lessons, plus complete luthier services that include guitar, string instrument and amplifier repair/restorations, we are also a great resource service. Below is a discussion on getting your guitar set-up to play its best and stay in tune.
Does your guitar not stay in tune while playing up the fretboard, or just not stay in tune? Well, due to how the guitar is designed/constructed and the dissonance of the tempered scale, your guitar will never achieve perfect tuning over all notes and chords. The well-tempered scale was formalized by Johann Sebastian Bach in his "Well-Tempered Clavier", marking the beginning of the current circular temperament for music. Bach's solution to the limitations of the "Just Scales"intervals was to average out their discords allowing all 12 keys to be played on the same instrument without retuning. This also allowed for modulating keys and transposing with a single instrument. Since the unison/octave notes are the only intervals which are pure, they should be used for comparison of tones when tuning. 3rds and 6ths are sharp, and 5ths are slightly flat. Modern guitar tuning has evolved and settled with standard tuning of E A D G B E, with A=440Hz.
Nearly all new stringed instruments can and should be set-up to not only play with ease, but to play in tune, to the best they can, the same as a fine-tuned piano. Most electric guitars now have adjustable saddles in the bridge, and many acoustics now have compensated saddles both of which aid in playing in tune as one plays up the neck at the higher fret positions. Depending on the player's style, the final height adjustment is fine-tuned by feel. Also, a guitar with the same measured string clearances will not feel the same as another guitar with the same specs due to differences in fret size (height/width of the frets), scale length, string gauge, fretboard radius, width at nut, neck profile (shape), even what the instrument is made from, especially the fretboard (ebony/rosewood verses finished maple fretboards).
However, even an instrument that has been professionally set-up will change with time and wear causing tuning problems, playability issues and buzzing. Temperature shifts, humidity changes, fret wear under the strings, an undesired bow in the neck, and loose parts, even changes in tuning modes (such as dropped D, half or whole step down tuning) can all cause tuning and playability issues. Also a fretted note that does not ring out (a dead note) may indicate a loose fret which can be corrected by properly setting the fret height and gluing it in place, re-seating the fret, or refretting the instrument. Pickups too close to the strings can magnetize the strings causing sound wobble with major tuning issues. Additionally, electric instruments and their corresponding amplifiers will over time develop scratchy pots, corroded or damaged jacks, and failures such as tubes, speakers, electrolytic capacitors or other internal parts. Even the simple instrument cable is often subject to abuse and damage.
What can you check for? If the pitch of an open string and its 12th fret are not identical, then your intonation is off. To check the string height at the nut, hold each string at the 2nd fret and look for a sliver of light between the top of the 1st fret and the bottom of that string. Is your instrument's neck straight? While holding the strings at both the 1st fret and at the fret body, you should see a gap, the thickness of about 4 to 5 sheets of paper, at the 6th/7th fret, and decreasing but never touching all other frets between the two fretted points. Any more or less and the neck needs adjusting with the truss rod or heat straightening. Wondering if your string height is good? The saddle should be set (on average) to give a string height of 4/32"to 3/32"for acoustics and 3/32"to 2/32"for electrics (measured at the fret body between the top of the fret and the bottom of the string), as measured from bass to treble, following the curve of the fret board (2 pennies = 3/32", 1 nickel = 2/32"). If you are experiencing any issues in the playability or tuning of your instrument, it's time to see a knowledgeable and experienced Luthier (stringed instrument builder/repairman).