The tonic is “home” on the musical journey. A work begins with the tonic (usually), travels through tonal adventures, and finally returns home — giving a sense of completion and rest.
The tonic triad (tonic + third + fifth) is the most stable chord in any key. If a work ends on something other than the tonic, the listener feels “unresolved” and “suspended” — a tool modern composers use to create the effect of an open ending.
The tonic gives its name to the key itself: when we say “a work in D major,” D is the tonic. The whole modal hierarchy is built around it: the subdominant leads away from home, the dominant creates the tension of return, and the tonic grants release. Composers of the 20th century deliberately weakened or wholly avoided the sense of a tonic, in order to strip music of its habitual pull and open new, “centreless” spaces of sound.
