Tonic

The tonic is the note a key is named after. A song in C major uses the C major scale (C D E F G A B), and C is its tonic, step 1 of 7. A song in A minor uses the same white keys with A as home, so its tonic is A and its tonic chord is Am (A C E). Roman numerals write the tonic chord as I in a major key and i in a minor key.

You can hear the tonic without knowing its name. Play up the C major scale and stop on B: the line hangs in the air, because B sits a half step below C and leans into it. Add the top C and the phrase is done. Chords work the same way. Play C, F, G and stop, and you are left waiting. Play C again and the loop closes. Most songs end on the tonic chord, so the last chord is a good first guess when you try to find a song’s key by ear.

The other scale steps have names too. Step 5 is the dominant, and V to I (G to C) is the strongest pull back to the tonic. Step 4 is the subdominant, and IV to I (F to C) is the softer “amen” landing. Both moves are cadences.

Tonic vs root

People often say “root” when they mean the tonic, and in casual talk that works. Strictly, the root is the note a single chord is built on and named after, so G is the root of the G chord (G B D). The tonic belongs to the whole key. In C major, the G chord has G as its root while the tonic stays C.

You meet the tonic in every key name and every Roman numeral chart. Choosing one for your own song is what picking a key comes down to, and the chords in C major page lays out all seven chords built on the C major scale.