One of the fundamental concepts underlying Western music is harmonic function. Within this system, chords fall into one of three categories based on how their notes relate to the tonic, and the harmony of a song is guided by an underlying framework. The tonic or "home" chord sounds the most stable and at rest in the context of the key, and as such, serves as the starting and ending point of most musical works. From the tonic, songs typically move to predominant chords, which introduce new sounds while facilitating a departure from the stability of the tonic chord. Next, the harmony typically moves to a dominant chord, creating tension that ultimately resolves back to the tonic chord. The return to the tonic brings a sense of finality and a return to a state of stability and rest. This tonic-predominant-dominant-tonic structure is fundamental to Western music and can be observed on both a larger, overarching scale and a smaller, phrasal scale.
This formula imitates the literary archetype of the Hero's Journey, in which the protagonist ventures out into the unknown, confronts challenges, and returns home. The tonic chord can be said to represent the home base or the starting point, where the hero's journey begins. The predominant chord then represents the call to adventure, where the hero is departs from home and embarks on a journey. The dominant chord represents the climax or the point of highest tension in the hero's journey, where the hero faces their greatest obstacle. Finally, the return to the tonic represents the resolution or the hero's return home, having completed their journey.
Harmonic reduction is a process which examines the overarching harmony and analyzes sections of chords as singular chords, or according to the section's broader harmonic function. This process can take place any number of times, as pieces can be reduced down to fewer and fewer chords. At a fundamental level, almost every piece, reduced to a singular chord, would consist of the tonic chord.
Because of this, one could make the argument that there is no point in leaving the tonic, especially as it is the most stable and consonant chord. However; while staying on the tonic provides a sense of stability, remaining there indefinitely lacks interest and novelty. Without the hero's decision to leave home and pursue adventure, there would be no story to tell. In the same way that venturing out of our comfort zones is what produces the most memorable life experiences, moving beyond the tonic is what creates the most compelling music. Just as in life, where although we come from and return to the same place at birth and death, it is what we do in the space between which gives meaning — in music, though we depart from and return to the same harmonic center, it is what we choose to do between tonic and tonic which creates meaning.
The role of the tonic chord provides identity and context to music in the same way that one's identity and sense of self shapes the way one interprets one's life. As people go through different experiences, they can come to understand themselves in new ways, leading to a shift in identity. While the idea of the tonic remains consistent throughout a piece, through the use of different harmonies, the key area (and thus the chord which functions as the tonic) can transform. This kind of movement to a new key area within a piece is known as modulation. Modulation can be a smooth or jarring process, just as people's understanding of themselves can change gradually or suddenly. In the same way that a shift in one's understanding of self can lead to a richer way of experiencing and engaging with life, modulating can lead to new musical experiences and opportunities for expression.
Since pieces can be transposed into any key and communicate an almost identical idea, it is not the exact chords, but rather the relationships between the chords which create the meaning in the piece. Just as people from extremely different backgrounds and life histories can connect over the deepest parts of the human experience, it is the fundamental relationships within a piece of music — regardless of its key or individual notes — which create its power and meaning.