Tonic
The tonic is the first degree of the scale, the note a key is named after. It is the home of the scale: the note where the music comes to rest. In C major the tonic is C, in A minor it is A.
A scale starts on the tonic and ends on it. Most tunes end on the tonic too – that is what makes an ending sound finished. If a tune stops on another note, say the seventh degree B, it is left hanging in the air and pulls towards the tonic.
The tonic is also a chord: the triad built on degree I. In C major that is C (C–E–G), in A minor Am (A–C–E). It is marked I (or T, for tonic). The other two main chords – the subdominant (IV) and the dominant (V) – are defined by their relation to the tonic: IV steps away from home, V pulls back.
The first word of a key’s name is its tonic: G major has the tonic G, D minor the tonic D. Relative keys share a key signature but have different tonics: C major and A minor.
What it sounds like
The tonic sounds calm and stable, like a full stop at the end of a sentence. Every other note of the scale pulls towards it, more or less strongly.
Where it is taught: the studio’s interactive textbook, chapter 6, “The C major scale” and chapter 9, “Triads: C, F, G”.