Seventh chords are constructed from "stacking thirds" intervals.
Usually, a very common seven note "diatonic" scale is used to derive the chord from.
The main ones to get these chords from:
- major scale
- melodic minor scale
- harmonic minor scale
To get your seventh chord, pick a starting note.
Next, skip one note and keep the next one.
Do this until you have 4 notes.
The foremost scale where we get the diminished 7th chord from, is the harmonic minor scale.
The 7th note of this scale has the dim7 chord, and you get it by "skipping 1 note each time".
Example: A harmonic minor
INTERVALS & NOTES:
1 - 2 - b3 - 4 - 5 - b6 - 7
a - b - c - d - e - f - g#
Starting from the G# note:
1 - b2 - b3 - b4 - b5 - b6 - bb7
g# - a - b - c - d - e - f
Skipping every other note, we're left with:
1 - - b3 - - b5 - - bb7
g# - - b - - d - - f
The main reason why we get these weird numbers like b4 and bb7 is because we're not "allowed" or "supposed" to repeat ANY number. So we're not writing a b2 and a #2, or a b4 and a normal 3, but raising the number each time.
Personally, I look at the "b4" as a regular "3".
Personally, I look at the bb7 either way, as a "natural 6" or a lowered b7.
This is reflected by how I decided to put the interval in mode 7 of the A harmonic minor scale: https://fretboardknowledge.com/guitar/kb/a-harmonic-minor/
We have to talk about Diminished Chords
