Metalocity

The minor key is by far more frequently used than the major key in metal music.  Part of the attraction of the minor key is its characteristic intensity and flexibility of usage.  The major and minor keys are often summarized in emotional terms—“happy” and “optimistic” for major, “sad” and “forboding” for minor.  However, to think of these keys in such limited terms is carelessly simplistic; a wide range of emotions and atmospheres can be conveyed by either key.  It is accurate, though, to recognize that the minor key is distinguished by musical qualities that lead quickly to dissonance and deviance from more traditional harmonic and melodic practices—qualities that coincide with the compositional philosophy of metal.

The key of A minor is an appropriate place to start in examining the minor key, as it does not contain any immediate accidentals (i.e. sharps or flats) and it appears with regularity in metal.  The key of A minor consists of the notes A – B – C – D – E – F – G – A, with the scale starting over on the latter A.

The notes of a key are also described in terms of scale degrees.  Scale degrees are numerical indications of the relation of each note to the tonic scale.  The tonic note serves as scale-degree one; the second note, scale-degree two, and so forth.  Scale degrees are indicated with Arabic numerals with a circumflex sign (^) hovering above.  In A minor, the scale degrees are as follows:
Scale-degree one, the tonic note, is the most important note in the scale–it is heard as the main destination point within the melody, to which all other notes tend to move.  Scale-degrees three and five are also perceived as destination points within a melody, though not with the same finality and stability as the tonic.

One of the main characteristics that distinguishes the minor key is the pattern of spacing in between the notes.  Successive notes are typically separated by an interval of a half step (the smallest unit of measure) or a whole step (equal to two half steps).  The intervals in between the notes of A minor as follows:

The locations of the half step relationships greatly affect the character of a key, for the half steps bring an intensity that drives melodic movement.  Imagine that notes are like magnetic poles, and the attraction between them becomes stronger as they are moved closer to each other.  In this manner, notes that are separated by a half step will tend to one another; they will move smoothly and forcefully.

In minor, the half step relationship between scale-degrees two and three gives the third scale degree a strong tendency to move to two.  In other words, it is more easily “pulled down” than it is “drawn up” (since the relation between scale-degrees three and four is a whole step).  The fact that the second scale degree is followed downward by the tonic add greatly to this gravity, because the tonic is perceived as the final destination point for the melody.  Hence, the first three scale degrees are structured in a manner that lends easily to a descending melodic line—scale-degree three descending through two and continuing down to one.

The other half step spacing in minor occurs between scale-degrees five and six.  With the fifth scale degree serving as a destination point, the sixth scale degree has a strong tendency to move down to it.  Again, this quality lends to descending melodic patterns with scale-degree six moving down to five.

Of course, these are only tendencies and not rules; melodies may move in any direction from these points.  The ultimate effect is that the two points where the notes are separated by half steps are points of great intensity, and in the minor key these points are advantageous to descending melodic lines to the first and fifth scale degrees.