To understand the idea of a sus chord, you must have a good grasp on major chords and scales. Chords are constructed from the notes of a particular scale. Major chords are constructed from major scales. A major scale consists of eight notes. The C major scale is C (1), D (2), E (3), F (4), G (5), A (6), B (7) and C (8). The second C is an octave above the first C. A major chord is constructed with the root note, the third and the fifth. A C major chord is C, E and G.
The term “sus” is a shorthand abbreviation for “suspended,” implying the sense of suspense or tension. A sus chord sets up a feeling of tension that leaves the listener waiting to hear a resolution. The chord is constructed by raising the third note of the chord a half-step or lowering the third note a whole step. In other words, the fourth note replaces the third, making it a sus4 chord, or the second note replaces the third, resulting in a sus2 chord. For example, in a Csus4 chord are the notes C, F and G; F replaces E. A Csus2 chord is C, D and G, with D replacing E.