If a guitar instructor strums the E, Em, and E7 chords, one after the other, the concept he is demonstrating is likely be
C. Chord quality
The instructor is trying to demonstrate the difference in sounds for different types of E chords. How different they in terms of sound and fingure position on frets.
Giotto di Bondone is the painter who formed his own technique in painting. He traded the Bryzantine Techniques into his style which is earthly, natural style Naturalism as it is called.