Chords are constructed by stacking intervals of the same name. The most common way is to stack 3rds. But there are also chords based on stacking 2nds (Clusters) and 4ths (Quartal chords). When you stack 3rds to get a 3-note-chord, you can stack - a major 3rd with a minor 3rd > this results in a major triad - a minor 3rd with a major 3rd > minor triad - a minor 3rd with a minor 3rd > diminished triad - a major 3rd with a major 3rd > augmented triad You can stack more notes. Stacking 3rds means skipping every other note on a scale. So when you take a major scale, and start with C, you take C, skip the next note D, take E, skip F, take G, skip A, take B, skip C, take D, skip E, take F, skip G, take A, skip B. So, by doing that, you get CEG > C maj CEGB > C maj7 CEGBD > C maj7/9 CEGBDF > C maj7/9/11 CEGBDFA > Cmaj7/9/11/13 When you take the same C major scale, but start building the chord on the 2nd note, you get DFACEGB > Dm7/9/11/13 The different intervals in chords have specific functions: - 1 and 5 build the foundation (in a band, you can leave them out of your chords, because the bass player will take care of them) - 3 and 7 define the function of the chord (like minor7 or major7) - 9, 11, 13 provide additional colours Now about the diminished chords: When you stack 3 notes of the C major scale starting from B, you get BDF - a Bdim diminished triad. But - when you add the next note - BDFA - you get a Bm7/b5 chord, not a diminished 7th chord. Diminished 7th chords are based on the diminished scale. Which is just whole step - half step all the way through. Diminished 7th chords are constructed by stacking minor 3rds. C-Eb-Gb-Bbb, which you can (mis)spell as C-Eb-Gb-A, for clarity. Every note can be the root, so this chord is Cdim7, but also Ebdim7, Gbdim7, and Adim7. For added craziness: Check out Pat Martino's approach to chord construction. There's a free pdf, and also books and a Truefire course. He derives all triads from the augmented triad, and all 7th chords from the dim7 chord.