In music, the tonic is the first scale degree of a diatonic scale (the first note of a scale) and the tonal center or final resolution tone that is commonly used in the final cadence in tonal (musical key-based) classical music, popular music and traditional music. The triad formed on the tonic note, the tonic chord, is thus the most significant chord in these styles of music. More generally, the tonic is the pitch upon which all other pitches of a piece are hierarchically referenced. Scales are named after their tonics, thus the tonic of the scale of C is the note C.
In very much conventionally tonal music, harmonic analysis will reveal a broad prevalence of the primary (often triadic) harmonies: tonic, dominant, and subdominant (i.e., I and its chief auxiliaries a 5th removed), and especially the first two of these.-- Berry (1976)
The tonic is often confused with the root, which is the reference note of a chord, rather than that of the scale. Most chords (e.g., ii, IV, V) are not tonic chords.
The tonic chord is represented with the Roman numeral I (if major) or i (if minor). The other types of chords are given different Roman numerals, such as (in a major key) ii for the chord built on the second scale degree, IV for the chord built on the fourth scale degree, V for the chord built on the fifth scale degree, and so on.
In western European tonal music (key-based music) of the 18th and 19th centuries, the tonic center was the most important of all the different tone centers which a composer used in a piece of music, with most pieces beginning and ending on the tonic, usually modulating to the dominant (the fifth above the tonic, or the fourth note down from the tonic) in between. Two parallel keys have the same tonic. For example, in both C major and C minor, the tonic is C. However, relative keys (two different scales that share a key signature) have different tonics. For example, C major and A minor share a key signature that feature no sharps or flats, despite having different tonic pitches (C and A, respectively).
Tonic may be reserved exclusively for use in tonal contexts while tonal center and/or pitch center may be used in post- and atonal music: "For purposes of non-tonal centric music, it might be a good idea to have the term 'tone center' refer to the more general class of which 'tonics' (or tone centers in tonal contexts) could be regarded as a subclass." Thus a pitch center may function referentially or contextually in an atonal context, often acting as axis or line of symmetry in an interval cycle.Pitch centricity was coined by Arthur Berger in his "Problems of Pitch Organization in Stravinsky".
The tonic diatonic function includes 4 separate activities or roles as the principal goal tone, initiating event, generator of other tones, and the stable center neutralizing the tension between dominant and subdominant