Each major and minor scale has seven notes which are determined by the scale’s pattern of intervals from the starting note. Scale degrees refer to notes’ positions within a scale, and each degree of the scale has a unique function relative to the tonic, or tonal center, influenced by the consonance or dissonance each note forms with the tonic.
An interval’s level of consonance or dissonance is determined by the ratio between the two pitches’ frequencies. Intervals which have smaller frequency ratios are typically perceived as more consonant or “stable”. The notes of an octave, in which the Hertz of the higher pitch is twice the Hertz of the lower pitch, have a ratio of 2:1, making it the most consonant interval. The next smallest ratio, 3:2, represents the relationship between a note and its fifth (counting up the degrees of the major scale); and the ratio 4:3 represents the relationship between a note and its fourth. Because of their acoustic stability, octaves, fifths, and fourths have been termed “perfect intervals”. These intervals are the first three intervals of the overtone series, a sequence of higher intervals that naturally resonate when any pitch is produced.
Intervals of increasingly complex frequency ratios are generally perceived as increasingly dissonant. The additional degrees of the major scale, listed by increasing dissonance over the tonic, are 6, 3, 7, and 2.
Because the tonic functions as a tonal center, the notes closest to it within the scale provide an increased inclination towards resolution to the tonic. The seventh degree of the major scale creates the most tension with, and consequently the most attraction to, the upper repetition of the tonic, as it is only a semitone away.