Major
Major is the bright-sounding mode. A major scale follows the pattern whole, whole, half, whole, whole, whole, half, and a major triad has a major third (four half steps) at the bottom.
The simplest major scale is C major: the white keys from C to C. Its half steps fall between E and F and between B and C, where there is no black key between the white ones. This pattern – two whole steps, a half step, three whole steps, a half step – is what makes a scale major.
The same pattern can start on any note. From G it needs F♯ (G major), from F it needs B♭ (F major). The key signature tells you which: no signs is C major, one sharp G major, one flat F major. On European scores you will also see C-dur, G-dur, F-dur.
A triad can be major too. C–E–G: from C to E is four half steps, a major third; from E to G three, a minor third. A major third at the bottom makes the triad major; in a minor triad the minor third is at the bottom. Major sounds bright, minor darker.
What it sounds like
Major sounds bright, calm and firm – most listeners call it cheerful or happy. Compare it with minor: the same tune in minor sounds darker.
Where it is taught: the studio’s interactive textbook, chapter 6, “The C major scale” and chapter 9, “Triads: C, F, G”.