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The problem it solves
Before understanding degrees, scales and functions you have to feel which note acts as the centre of gravity. Without identifying the tonic you cannot tell where a phrase rests or how the key is organised around it.
Detailed theory
Key idea
The tonic is the point of maximum rest: the "home" note toward which the music tends to return.
The dominant (chord V) builds tension that resolves toward the tonic; the leading tone (degree 7) leans into it.
Understand it
In a key there is one note that rules above the rest: the tonic. It is the point of maximum rest and stability, the place where a phrase feels finished. The whole web of relationships we call a key is organised around this central note.
Melodies and harmonies feel resolved when they reach the tonic. That is why you recognise it by the sense of arrival: if you hum a song and stop too soon, you notice something is "missing"; when you end on the tonic, everything is closed.
Two degrees work to lead you toward the tonic. The dominant (degree 5, chord V) builds tension that demands to resolve toward the tonic, and the leading tone (degree 7), just a semitone below, leans into it strongly. This play of tension and resolution is what gives tonal music its direction.
An analogy: the tonic is like gravity or the centre of a solar system. Every other note orbits around it and, sooner or later, is pulled back home. The further you move away (through the dominant, through the leading tone), the stronger you feel the pull toward home.
Watch out: the tonic is not necessarily the first note of a piece, but the one that gives a sense of arrival. A song can start on any degree and end up establishing the tonic by repetition, position and resolution.
Chord progression
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I-V-I in C major: it leaves the tonic, passes through the dominant and returns. The tonic organises the whole key.
How to recognise it
How it's written
The key signature often points to the tonic: it indicates which key the piece establishes. In analysis, the tonic is degree I and the reference chord; look at where the phrases tend to resolve to confirm it.
How it feels
Hum a melody and stop it on different notes: the tonic is the one where everything feels finished and at rest. If you stop on another, you will notice the music still "wants" to continue toward home.
Common mistake
Thinking the tonic is always the first note of a piece: it is the one that gives a sense of arrival, which can appear at the end.
Confusing the dominant with the tonic: the dominant pulls and demands resolution; the tonic is the rest where that tension resolves.
Try it
Play a C major scale and always end by returning to C: you will notice it is the only note where everything is at rest.
Play the G major chord (the dominant V) and then the C major one (the tonic I): feel how the tension of V resolves toward the tonic.
On the instrument
Tension curve
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V→I in C major: the dominant (G) builds tension and resolves toward the tonic (C), the resting point.
Staff & keyboard
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C as the tonic of C major: the centre of gravity toward which all the music of the key tends to return.
Where it's used
- Finding the key of a song
- Identifying which note the music "wants to return to" to work out its key.
- Closing a phrase
- Knowing which sound gives rest when you want to end a melody with a sense of arrival.
- Feeling the resolution
- Recognising how the tension of the dominant (V) resolves toward the tonic (I).
Examples
Staff & keyboard
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A short phrase descending G-F-E-D and closing on C: reaching the tonic gives the sense of final rest.
Mini test
Check that you've got it.
0/10 answeredQuestion 1/10
What is the tonic of a key?