These major or minor chords contain three different notes: a root (eg the E note in E), a 3rd interval up from the root (eg the G# note in E) and a 5th interval up from the root (eg the B note in E).

This example of an interval of a third is in the bass clef. ... D minor (notes are D, F and A) III - E minor ... A diminished 7th is built on the 7th note of the scale.

From The Doobie Brothers to The Smiths, James Brown to Metallica, 7th barre chords are fundamental for playing with more color and sophistication ...