Melody is a universal human phenomenon, traceable to pre-historic times. The origins of melodic thinking have been sought in language, in birdsong and other animal sounds, and in the crying and playing of young children. When we hear a sound, we are hearing the air vibrate. Humans can hear as few as 20 vibrations a second or as many as 20,000. Back up Harmony is the sounding together of two or more pitches.
Chords are the individual units of harmony.
Their function is determined by the key. Key is the basic scale of a composition. It is named for its key note, the first note in the scale. Diatonic Harmony refers to the use of chords and pitches from within the key only. Chromatic Harmony refers to the use of chords and pitches from outside the key.