No, he is referring to methods of tuning. A piano is tuned to equal temperament, where the distance between every half step is exactly the same. However, this is not actual "true tuning" but it's very close. True tuning of chords would be following their harmonic ratios (ex 1:2 for an octave). You know that sound when a fifth is perfectly in tune and they almost blend together, like a hug? That sound is possible with almost every interval. When tuning double stops, you should use just intonation, where major thirds are smaller, minor thirds are bigger, etc. When playing melodies, we use Pythagorean tuning, a tuning method where half steps are emphasized. For example, you may play the C# of a D major scale higher than normal to emphasize the leading tone, or the F# higher to emphasize the major scale. You can test this by playing D E F#, and then play the F# with the A. Is your F# in tune with the A? Probably not, but it sounds in tune for the SCALE. If you play the F# in tune with the A, it will sound LOW for the scale, because our ears want to hear a HIGH F# for the scale. This is why tuning a chord to a tuner is not really accurate!!! If you're tuning a third D and F#, you can check the D with the tuner, but you should tune the F# with your ears until the beating disappears! TL;DR: Strings use two tuning methods, Just intonation for chords and double stops and Pythagorean for scales. Read above for more detail about these methods.