A Suspended 4th
sus4
0 - 5 - 7Root + perfect 4th + perfect 5th
About A Suspended 4th
Asus4 replaces the third of an A major chord with a fourth — A, D, E — and unlike its sus2 relative, this chord leans. The D sits a semitone above the C♯ it replaced, and the ear wants it to fall back. That expectation is the whole reason the chord exists.
The notes
A, D, E. No third, so no mode; but the fourth creates a tension the second does not. Play Asus4 and then A, and the resolution is audible even to an untrained listener.
On guitar the shape adds one finger to the standard A: the little finger at the third fret of the B string. Lifting it resolves the chord, which is why the figure is so common in strumming.
How it is used
As a delay. A bar of Asus4 before a bar of A stretches the arrival, and the technique appears in folk, rock and worship music in roughly equal measure.
It also serves as a substitute for the dominant in modal writing, where the missing third keeps the harmony ambiguous while the fourth supplies movement.
The same figure works with a capo anywhere on the neck, which is why it survives transposition better than most ornaments: the shape and the resolution stay identical whatever key the song ends up in.
Where it shows up
In the key of A as an ornament of the tonic, and in the key of D as a suspended dominant — the sound of a chorus about to start.
The alternation between A, Asus4 and Asus2 covers three chords with one hand position and two fingers, which is why so many acoustic songs use all three within a single bar.