The quality of a chord is determined by what notes follow the root of the chord. All qualities follow a formula to define them. We can describe the form by referring to the chord's intervals. Some examples of chord qualities are Major, Minor, Dominant (7), Major 7th, Minor 7th, etc.

As explained in building triads we see that chords are built from thirds. To get an idea of what we mean by thirds, we can examine the C Major scale below.

If we switch out the note labels for interval labels, we see how each scale degree is represented by a number.

When looking at the major scale, we see that each degree of the scale is separated by either a min 2nd or a maj 2nd interval. The next example is a C Major chord. The interval labels illustrate how the notes of chords are usually separated by thirds.

Remember, 'thirds' simply mean that the distance between two notes are either a major 3rd or a minor 3rd. Please refer to the interval chart to get a better idea. (a minor 3rd is 3 half steps, while a major 3rd is 4.)

Triads

intervals

triads in C

Seventh Chords

intervals

Seventh Chords in C

Tensions

intervals

Tensions in C

When we think about what Quality means, you should realize that it's really just a way to refer to every permutation of ordering thirds in different ways.