The tonic is the first note (or degree) of a major or minor scale, also known as a keynote because it is the note from which the key takes its name; for example, the key of D.

The tonic triad is the triad built on that note; for example, the C-major triad in the key of C. The close companion of the tonic triad is the dominant, built on the fifth note of the scale; in conventional tonality, it often leads back to the tonic, in a progression is called dominant-tonic (or V-I).