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I'm just learning music theory and I already understand the scales formulas (minor, major, harmonic, melodic, pentatonic, modes, etc.) and chords formulas (min, maj, 7th, dim,etc) but what I can't understand yet is, if, for example, I'm working with D Major scale and I want to make the progression D-F#-B (I-III-VI) how do I know what types of chords those three notes (D-F#-B) can/should be?

For example, can do I know if i can those three notes on a 7th chord, or a diminished chord, or a suspended chord, and still be on the same scale? Or maybe one of those notes can be a 7th chord but the other two can't and why?

So basically my confusion is if there are specific formulas as well for the types of chords for every note on every scale (say that, in X scale the third note should always be 7th chord) or I'm just mixing everything?

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The vast majority of the time you'd take the scale and harmonize it in 3rds to produce a set of chords whose notes come from that scale.

By a 3rd, I mean an interval of 3rd that stays within the key. So in the key of D you'd have D to F# which is a major 3rd. But if you started from the 2nd degree, E, you'd get E to G which is a minor third. Remember that difference because it's why the I chord is major but the ii chord is minor.

So let's take the D Major scale:

D E F# G A B C# 1 2 3 4 5 6 7

Then, starting from each degree, we'll build a chord in thirds which is basically taking every other note and wrapping back around to the beginning as need be:

  • D F# A C#
  • E G B D
  • F# A C# E
  • etc

Those are the 7th chords but you could use one note less for triads or more for the extended chords. And if you know your chord formulas you'll start to see that for each degree there's a certain chord type that stays the same even when you switch keys:

  1. IMaj7 (DMaj7)
  2. iim7 (Em7)
  3. iiim7 (F#m7)
  4. IVMaj7 (GMaj7)
  5. V7 (A7)
  6. vim7 (Bm7)
  7. viim7(b5) (C#m7b5)

As for when you'd use which degree—how you'd make a progression—that's a larger question requires a look at functional harmony. But you can start with using these chord types and your ears.

You can do the same thing with other scales as well like say the Harmonic or Melodic Minor scales. Because the scales have a different structure you'll get different chord qualities for different degrees.

Lastly, you can harmonize in other intervals as well. For instance using 4ths is called "quartal" harmony and used in modern jazz quite a bit. But the vast majority of the music that you encounter will be using 3rds.

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If I understand correctly, you cannot always use notes from one particular scale to make chords in a particular key. User 37496 has given the main chords that do use only scale notes, but let's take a diminished chord as an example. Co= C, Eb, Gb, Bbb. That chord may well feature in the key of C major, but the only note from C is that note itself. It might be argued concerning the Bbb (A) but that doesn't matter - Eb and Gb are enough. Augmented, anyone? C+ = C, E, G#. There's no G# in C major. C7 ? There's no Bb in C, etc, etc.

So, in a lot of songs, the chords are not necessarily diatonic, for reasons above.

In your example, I guess you need 3 chords, to go with those notes, D, F# B. Well, any chord underneath a particular note will generally speaking have that note in it. Now, there's a big choice for each. For the D, there's D, Dm, G, Gm, Bb, Bm, Bo, Em7, Ebmaj7, E7 for starters. One or two will be the better ones, once it's decided which chord comes next, and bearing in mind what key the piece is in. The Ebmaj7 probably wouldn't be a great choice! D works well, and could be followed by D7 under the F#, and G under the B. Or - D; F#7; Bm. The 'rules' are there as a guideline, to say 'this has worked in the past, give it a try'. Rather than 'this is how it must be done'.

As far as the type of chord to use, it depends a lot on what function that has. For example, in D > D7 > G, D is tonic, D7 is dominant leading to G.

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