What is the significance of "em" in music theory and how does it impact the composition and performance of musical pieces?

In Music Theory, "em" is short for the relative minor key of a major key. It is significant because it provides a different emotional feel to a piece of music, often adding depth and complexity. Composers use the relative minor key to create contrast and evoke different moods within a composition. Musicians must be aware of the key changes to properly interpret and perform the piece, adjusting their playing style and emotional expression accordingly.