The shape of a melody is expressed across a specific scale. Certain pitches of the scale are more stable with the tonic, and as a result, exert a kind of gravity. Melodies feel pulled to these pitches because of the innate tendency of music to flow from tension to resolution. Generally, the most stable notes of a major or minor scale are degrees three and five, the notes of the tonic chord.

However, when we begin using different harmonies underneath our melody, the context changes. Think of it as a movie scene – even with the same actors and script, a change of set can cause what happens can be experienced entirely differently. When we move to a chord other than the tonic, the most stable notes become the notes of that chord, known as “chord tones.” While the tonic chord's tones still carry a degree of gravity, they are secondary to the pull of the current chord.