Learning how to harmonize a scale is a procedure that will allow you to figure out the possible chord formations as a consequence of using the notes from that tonality. In order to do so you just have to consider each note of such scale and build chords on top of it, be it chords in thirds, seconds, fourths, or any other kind.

Because It would be exhausting to cover every possible chord formation and their respective extensions, I will only be using triads or tetrads to harmonize the most common scales. We will be starting with the major scale and chords in thirds are going to be added on top of each scale degree. After that, the third intervals are going to be classified to find out what type of chord is formed over each degree:

As a quick tip; after you do this for the C major scale, as shown above, the chord qualities associated to each scale degree will remain the same for all the other major scales, like so:

Scale Degrees I ii iii IV V Vi vii VIII Chord Quality M m m M M m dim M

The only thing that changes is the actual name of the chord, which will be relative to root of the scale you are working with. The same goes for all the other scales, of course.

The next scale we will be harmonizing is the harmonic minor and the procedure is exactly the same, except that because of the raised 7th there is the occurrence of enharmonic notes between G# and Ab, meaning that there will be more than one type of chord quality for some scale degrees:

Again, the same procedure is going to be applied to the melodic minor scale:

Because this is a five-note scale, the pentatonic scale has a couple of degrees “missing” when compared to the diatonic scale with seven notes. This means that when we start building chords in thirds over the scale degrees, there will be some that won’t have the third interval in it because that interval doesn’t exist in reference to that particular degree. In such cases, we will adopt the first available interval above or below the missing third or fifth of the chord, meaning that we will end up with some suspended chords, where either the 2nd or the 4th can be suspended, and chord inversions:

As we will see, the half-whole diminished scale presents the possibility to form various types of chords, and not only diminished chords, due to other tones that can still be used as an alternate third of fifth for that scale degree. As a side note, this is the not the case with the whole-tone scale that produces only augmented chords. Further ahead, I will return to the chord materials produced with this diminished scale but for now, here is its harmonization:

In essence, the whole-half diminished scale has the same harmonization possibilities because it’s basically a half-whole diminished scale that begins on the second degree of the latter and thus it will not be represented here.

As a reminder, the harmonic layout of major, minor harmonic and melodic, pentatonic and diminished scales is the same for all tonalities since those will only be transpositions of what I presented here.

When you are using melodic material from a given scale, even a scale you may come up with, you can harmonize the melody with the chords derived from that scale harmonization. You may choose to use seventh chords, triads or any other type of chord extension or chord formation related to that particular scale material you are using.

As a final note, when we discuss the harmonic possibilities we have when we want to harmonize a melody, you will see that you are not stuck to the chords that go with a particular tonality and thus you will be able to enjoy great harmonic freedom.

Nevertheless, even seventh chords of the diatonic major scale can be used to create interesting harmonies and chord progressions. But this will only depend on your creativity when manipulating the materials at your disposal.