What is tonic key?

The tonic key is the primary key in a piece of music, serving as the home base or tonal center around which the composition is structured. It is established by the tonic chord, which is built on the first scale degree of the scale used in the piece. The tonic key provides a sense of resolution and stability, and melodies and harmonies often resolve back to this key throughout the music. In a major scale, for example, the tonic key is represented by the major chord built on the first note of the scale.