-- おさらい -- ★ Thirds are mandatory unless you're going for a suspended sound. To find the third, count up 4 half steps for a major or dominant chord, three half steps for a minor or dinimished chord. ★ Seventh add stability. To find the 7th, cound down a half step for major chords, two half steps for minor and dominant chords.
That is,
- 7th - Minor/Dominant = -2 Major = -1
- 3rd - Major/Minor = 7th --- <4th> --- 3rd Dominant = 7th -- <♭5>tritone --- 3rd Dmaj = D♭ D G♭ Dm = C D F D7 = C D F♯