Major third chords less dissonant with the third note lowered
 

https://youtu.be/Daw93bRHe4Y?t=422

Really nice lesson on ratios, intervals and to making the guitar resonate pleasingly.
If a songs composition has a prominent third note in a major scale it might help the the human ear to temper, lower third note string tuning.

Nothing new about that, those who don't rely on electronic tuners have been sweetening their tuning all along. The problem with tweaking the major 3rd note (the G# on the G string for an E major chord, for example, or the B on the A string and the open B string for a G major chord) is then the same string on a different fret for a different chord (the A major or C major) sound off. It's all a matter of compromise. Some players don't even seem to notice, but it drives me crazy!

Always a compromise. Get the D chord perfect and then A chord sounds bad.

This is why I prefer the sound of an open G chord with the B string fretted at the third fret rather than open, to eliminate that extra third ringing out. I can lighten the pressure on the fretted A string to sweeten that one a touch. Unfortunately that limits the licks I have available from that shape compared to the “pinky on the high E third fret with open B string” shape

I agree but it's all a trade off I suppose? Fingering of different chord shapes all depends on where I'm going next and what I want to get out of the progression. If that makes sense?

Many Dobro players do this as standard when playing GBDGBD, steel players and banjo players too.
Equal temperament is not always the best choice IMO, often tuning differently the gains outweigh the losses in harmony, also when playing open tunings.
It's why we put the DB0 and DBH and other such Sweetener presets in our tuners.

Stomp, what do "DBO" and "DBH" stand for? Thank you.

other tricks include playing voicings where the third is left out, or where the third is a larger interval from the root. Like a G chord where the root is on the sixth string, fifth string is damped, and you play a D on the second string instead of the B. Sounds very G-chord like, major chord, but no third. Or a C chord with a root on the fifth string, fourth string damped, and a high G on the first string. Or you play a Major 7 substitution for a major chord, so the third is juxtaposed against the 7 instead of the root.

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Wow. My ears must suck. Never noticed anything dissonant with the D chord shape, or any other chord for that matter....

I think during most chords in which the 3rd is fretted we unconsciously make minute adjustments with our finger pressure to get the sweetest sounding note that we can.

Major thirds in 12-tone Equal Temperament are 'off' by quite a lot. It's part of the price you pay for being able to play in all keys without rebuilding the instrument. If, like most of us, you grew up with 12-tone ET you don't tend to notice it. Until you do. Then the problems never go away. Sadly, it's mathematically impossible to construct a scale that has all of the fifths and octaves 'pure', so we're left with using some form of 'temperament'; applying a fudge factor that makes things at least tolerable.

It's explained here, if you read at the 3rd column in the bottom:
http://76.12.22.162/media/pdf/Peters...lipEnglish.pdf

The system of equal temperament is off by design. Equal temperament allows fixed pitch instruments such as piano to play in all keys, acknowledging that some intervals will be off. Same thing with fretted instruments, except it is easier for us who play those instruments to make quick adjustments as desired. The thing is that the intonation issue is most clear when playing triads or chords. The ear forgives single note play more easily. Non-fixed pitched instruments (i.e. brass, winds and strings) aren't restricted and while their instruments are designed to respond similarly, have the ability to manipulate the pitch on the fly. It should be noted that many piano tuners actually tend to raise the higher pitches on the piano (called "spreading the pitch) as pitches that are technically in tune tend to sound flat to the ear (a phenomenon that I don't understand, but was shown to me by a Steinway certified technician that worked for me for a time).

There are many different strategies for compensating for equal temperament. But much of the adjustments are compromises. Some folks, rather than lower the third will raise the 5th, which becomes less obvious in certain circumstances. Some guitar players use the James Taylor tuning method, which is both a partial compromise for equal temperament with some compensation for the raise of pitch due to string deflection (which is why we should always strive to tune while striking the string with the same force we will perform with).

One more thing though... With guitars there are two tuning forces at work: The vibration frequency of the stings and it's overtones, and the natural resonance of the guitar itself. A perfect example of trying to maximize that aspect is Taylor's v-class bracing. With the bracing configuration they are trying to allow the body of the guitar's resonance better match the strings' frequencies. I think for the most part they have succeeded... but in doing so brought out aspects of sound that change the balance of sound that our ears are used to and expect in acoustic guitars. I'll stop here.