While intervals can occur between two successive notes within a melody (melodic intervals), they can also occur simultaneously (harmonic intervals). When two or more pitches occur together, this creates harmony. Although some music features multiple melodies of equal importance, most music has a single main melody. Main melodies are traditionally sung by the highest voice part, known as soprano, and may be complemented by other melodic lines. Harmony, vocal or otherwise, which provides support, context, or enrichment to a melody, is known as accompaniment. The term accompaniment can also refer to anything which provides context for a melody — including rhythms.
While a melody may communicate a certain idea alone, adding even a single line of harmony can dramatically alter its character. In addition to the intervals melodic lines form with the tonal center, the intervals formed across harmony create specific meanings within the context of a song.
We have two ways of referring to intervals: one is by their frequency ratios, and the other is by how they appear within the context of the major or minor scale. The second way is much more common; however, it is simply a way of interpreting fundamental frequency ratios within the Western system of music.
Degrees of the scale, paired with the tonic, form intervals of their names. For example, the third degree of the major or minor scale, paired with the tonic, forms a third; and the sixth degree, paired with the tonic, forms a sixth. In addition to its number, each interval has a quality. Besides the fourth, fifth, and octave intervals, which are classified as “perfect,” each degree of the major scale, paired with the tonic, forms the major interval of its name. Shrinking a major interval by a semitone results in its corresponding minor interval. Perfect or minor intervals can be further modified by shrinking the interval by a semitone, changing its quality to diminished. Finally, expanding a major or perfect interval by a semitone changes its quality to augmented.
Major and minor intervals are considered “imperfect.” Imperfect intervals comprise two categories - consonant and dissonant. Although consonance and dissonance are a spectrum, intervals are classified into groups based on where they fall. Major and minor thirds and sixths are considered to be consonant intervals, and major and minor seconds and sevenths are classified as dissonant intervals.
Intervals of a given number and quality have a set number of semitones and can occur between pitches anywhere on the keyboard, not only in reference to a tonic. It is important to note that there are multiple ways of referring to the same intervals. For example, the interval composed of three semitones can be referred to as either a minor third or an augmented second. While both intervals have the same number of semitones and thus the same frequency ratio, what differentiates them is the way they are notated. If the interval is named using letter names a third apart — for example, F and Ab — it is considered a third. If it is named using letter names a second apart — for example, F and G# — it is considered a second. Intervals which use the same notes, but refer to them differently, are called enharmonic intervals. The specific way an intervals is named depends on its harmonic context.
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