Suspended and added-tone chords · From Zero to Mozart

A triad is root + 3rd + 5th. Swap the 3rd for a neighbour, or add an extra note, and you get chords that aren't quite triads — the open, floating, un-major-or-minor sounds all over folk and pop.

Suspended chords — the 3rd is missing

The 3rd is the note that makes a chord major or minor. Take it out and replace it with the note next door:

sus4 — replace the 3rd with the 4th (C–F–G instead of C–E–G). Tense, wants to resolve down to the major chord.
sus2 — replace the 3rd with the 2nd (C–D–G). Open, airy, ambiguous.

60 bpm

Bar 1 Gsus4 (G–C–D, the C wants to fall to B), bar 2 it resolves to G major (G–B–D), bar 3 Gsus2 (G–A–D, the A just hangs). Each triad is broken into its three notes so you can hear them. That sus4 → major move is one of the most common 'arrival' gestures in pop and hymns.

Added-tone chords — a triad plus one

Keep the whole triad and add a note, without stacking a full 7th:

6 chord — add the 6th (C–E–G–A). Sweet, nostalgic, a "soft landing" (old jazz standards end on these).
add9 — add the 9th (the 2nd, an octave up: C–E–G–D). Bright, modern, a bit cinematic.
6/9 — add both.

72 bpm

A C6 chord (broken) — C major plus the A. Then Cadd9 — C major plus the D. Neither has a 7th; each just has one extra colour note woven in.

Try it

Try

On the G and D strings, play a Gsus4: open G, 3rd finger C (G string), open D — rolled. Hold it, hear the tension. Now move the C back to B (high 2): G major. The suspension has resolved. That's the whole trick — a sus chord is a triad with one note "hung up" on the way to its proper place.

The usual mistake

Hearing a sus or add chord as a "wrong" triad with a bad note in it. They're deliberate: the sus chord withholds the major/minor decision for a beat of suspense; the add chord tints a plain triad without turning it into a full 7th chord. Both are about colour without commitment.