Harmony refers to the sound created when several notes play at the same time, and to the relationships in those combinations. Backing vocals stacked above or below a lead, chords supporting a melody, the thickness made by several instruments: harmony is the element that gives a song depth and color.
Relation to chords
A chord is a concrete group of notes that creates harmony. A C major chord sounding C, E, and G together is one harmony. Harmony is a slightly broader word that covers not only chords themselves but also the relationship between melody and chords and the motion of the voices.
How it differs from unison
Unison, playing the same notes together, means several parts sounding the same melody. In harmony, you stack pitches different from the lead. Unison tends to create thickness and unity, while harmony creates width and color.
Harmony in vocals
Layering another melody a third above or below the lead gives an easily recognizable harmony. Rather than stacking every note at the same interval, it is important to choose notes that fit the key and the chords of the song.
How to try it in a DAW
Start by duplicating the lead MIDI and moving it a little up or down. Adjusting the notes to fit the chords creates natural thickness. Trying just a short part, such as the end of the chorus, makes the difference easier to hear.
Keep each role clear
The more harmony you add, the more the lead can get buried. Keep the main melody clear and layer harmony only where it is needed, and it stays effective.
