Variation by its very nature is a tool for creating themes, since the repetition of elements of an initial idea makes that particular idea grow in importance and hence giving credence to its status as a “theme.” Variation is also, naturally, about transformation, and so as an initial theme is continuously transformed over multiple variations so does that evolution become one of the main features of the particular section(s) of music where the transformation occurs.
In Metallica’s works, the usage of variation for thematic purposes includes introducing and evolving ideas into a main theme (similar to a formal exposition), combining elements of multiple themes, and promoting the theme as the marker of a particular section or entire song. Evidence of the latter is found in “The God That Failed,” in which the theme is extended over the entire song and the variations that occur explore the theme via drastic alterations of the original idea. The following examples will explore the former two categories of thematic usage.
The introduction and evolution of a theme is a practice that occurs particularly in the intro section of a song. Over the course of the intro, elements of a theme are presented and undergo variation, eventually coalescing into the theme. One of the earlier examples of this practice occurs in the song “The Thing That Should Not Be” from Master of Puppets. The song begins with the simplest of motives, an ascent from a D power chord to an E power chord, with the E chord sustaining for nearly two measures. (This riff is facilitated by a “dropped-D” tuning, in which the low E string is tuned down to a D.)