Chords from the scale
In 3.1 you stacked 1, 3, 5 from the bottom of the scale. A diatonic chord is the same move started on any rung: take a note, skip one, take, skip, take, and never leave the scale. Seven starting rungs, seven chords, all made of nothing but scale notes.
Any rung, same recipe
The tinted keys are the scale, the only notes allowed, numbered by rung. Pick a home note, pick a rung, then stack the same skips as always: take the rung, skip one, take, skip one, take. Whatever comes out is that step's diatonic chord. Shapes mark the scale's own 1, 3, and 5: star, triangle, pentagon.
Only scale notes
That's what diatonic means: of the key. The scale is the palette: seven notes to build from, nothing else. Start a chord on D and count skips: the scale hands you F, not F♯. Hear the difference.
Quiz
1 / 3A diatonic chord is built from…