The major diatonic field
Seven chords born from a single scale, and the reason each quality lands exactly where it does.
Take a major scale. On each degree, stack two thirds using only the notes of that scale. The result is seven chords, and they explain why so many songs always use the same handful.
Stacking
On the first degree, C, stack by skipping notes: C, E, G. On the second, D: D, F, A. And so on, always skipping one.
What comes out of it:
Why the qualities land where they do
Nobody chose that the second degree would be minor. The scale forces it.
The D chord uses D, F and A. From D to F is three semitones, , a minor third. That is what makes the chord minor, and no other combination was possible without leaving the scale.
The seventh degree is the most interesting. B, D, F: a minor third and then a diminished fifth, . No other degree has that, and it is why the seventh degree sounds unstable in a way the others do not.
With sevenths
Stack one more third on each degree and the field looks like this:
Notice the fifth degree: is the only one with a major triad and a minor seventh. That pair produces an internal tritone, between B and F, and it is what asks for resolution. All the tension of Western tonal music fits in that interval.
The same thing in any key
The field does not change shape; it changes height. In F major, the qualities land on exactly the same degrees:
That is why musicians think in degrees and not in chord names. Learn it once, and it serves all twelve keys.
A spelling trap
The fourth degree of F major is B flat, not A sharp: the same sound, a different letter. The scale has already used the letter A on the third degree, and using it again would leave the field with no B.
That rule, each letter appears once, is what determines the entire spelling of accidentals in every major scale.
What to take from this
The diatonic field is not a table to memorise. It is a consequence: given a scale and the rule of stacking thirds, the seven chords and their qualities are already determined. Understanding that is the difference between remembering and knowing.