Third
A third is an interval of three letter names, such as C–E. A major third is four half steps, a minor third three. A triad is two thirds stacked up.
From C to E there are three letter names, both ends included: C, D, E. That is a third. On the keyboard a third is every other white key: C–E, D–F, E–G. On the staff both notes sit on neighbouring lines, or in neighbouring spaces.
Thirds are not all the same size. From C to E is four half steps: a major third. From D to F is only three: a minor third. Both are thirds, because both span three letter names, but they sound different: the major third brighter, the minor third darker.
The third is what tells major from minor. A triad is two thirds, one on top of the other: with a major third at the bottom (C–E–G) the triad is major, with a minor third (A–C–E) it is minor. The first leap of “Greensleeves” is a minor third up.
What it sounds like
A third sounds sweet and warm; it is the most agreeable of the small intervals. A major third is bright, a minor third darker and a little sad.
Where it is taught: the studio’s interactive textbook, chapter 9, “Triads: C, F, G” and chapter 15, “Intervals: unison to octave”.