It is common, however, to alter those triads slightly and arrive at chords that are derived from stacked thirds, but that contain other intervals. This is done by altering the second note in those triads – the 3rd, whether it is a Major 3rd or a minor 3rd. These new chords are called suspensions, and there are two types of them.
First, there are suspended chords in which the 3rd is lowered to the 2nd. These are called suspended 2nd chords, or just sus2. Suspended 2nd triads – If you begin with a Major or minor triad and lower the 3rd to a Major 2nd, then you will have a sus2 triad. It consists of a root, a Major 2nd and a Perfect 5th, and it is built from a Perfect 4th stacked on top of a Major 2nd. The chord formula for a sus2 chord is: 1 2 5. This means that the notes of Csus2 chord would be: C D G (we just take the 2nd note instead of the 3rd from the C major scale).
The second kind of suspended triad is one in which the 3rd is raised to a 4th (rather than lowered to a 2nd). These are called suspended 4th chords, and are commonly used in jazz as well as in rock and pop to add specific color to Major and minor triad progressions. Like sus2, the suspended 4th triad is neither Major nor minor. Beginning with either a Major or minor triad, sus4s are built by raising the second note in the chord (the Major or minor 3rd) up to a perfect 4th. It is built from a Major 2nd interval stacked on top of a Perfect 4th (the opposite of sus2), and it is composed of a root, a Perfect 4th and a Perfect 5th. The chord formula for a sus4 chord is: 1 4 5. The notes of Csus4 chord would be: C F G.
It is also possible to talk about suspensions of diminished and augmented chords, although these are very rarely used. In these cases, the suspended chords have the same qualities as before, only the 5th is either flattened (in the case of a suspension of a diminished chord) or sharpened (in the case of a suspension of an Augmented chord). These chords are shown in the following way: dimsus4 (1 4 b5), dimsus2 (1 2 b5), augsus4 (1 4 #5), augsus2 (1 2 #5). The dimsus4 is extremely dissonant because there is only a half-step difference between the Perfect 4th and diminished 5th.