Functional Harmony: Tonic, Predominant, Dominant
Tonic, predominant and dominant describe what a chord does to a phrase rather than what it is. This is how the three harmonic jobs divide the work.
A chord chart tells you which chords a piece uses. It does not tell you why the piece moves. The labels tonic, predominant and dominant answer the second question: they describe what a chord does to the phrase it sits in, not what notes it happens to contain.
The clearest test uses three chords. In C major, take C, F and G. Played C–F–G–C they sound like a statement, a departure and a return. Played G–C–F–G the same three chords sound adrift, because the ear reads every chord against where it believes home to be and where the music seems to be heading. Function is that reading.
Three jobs, not three chords
Every diatonic chord in a key can be assigned one of three jobs.
Tonic is the point of rest. It states the key, and it is the position a phrase measures its distance from. In C major the tonic job belongs to the C chord, but not to C alone: A minor and E minor share enough of its notes to stand in for it.
Predominant, also called subdominant, leaves home and aims at the dominant. F and D minor carry scale degree 4, the note that will eventually fall to scale degree 3 over the dominant.
Dominant makes the return to the tonic sound required rather than merely possible. The leading tone and scale degree 4 are both present in the dominant seventh, G–B–D–F, and in the diminished triad on the leading tone, B–D–F; in both, the two notes form the tritone that has to resolve.
| Function | In major | In minor | What the chord does |
|---|---|---|---|
| Tonic | I, vi, iii | i, VI, III | states or restates the key, provides rest |
| Predominant | IV, ii, ii6 | iv, ii°6, VI | leaves the tonic and prepares the dominant |
| Dominant | V, V7, vii°, viiø7 | V, V7, vii°7 | makes the tonic’s arrival feel necessary |
The table names chords, but the job belongs to the context. A G major triad is the tonic of G major and the dominant of C major. In C major, A minor can be a soft rest, with vi standing in for I, or it can be a momentary new home after a D major chord has pointed at it. The same object, two jobs, decided by the phrase.
The engine underneath: roots falling by fifth
The progressions that sound strongest move roots down a fifth, which is the same as up a fourth. A minor, D minor, G, C: three descending fifths in a row, arriving home. Each root becomes the fifth of the chord that follows it, so the bass makes large, unambiguous steps while the upper voices barely move. The harmony travels a long way and the voices do very little.
The last stretch of that chain is the part that matters most: predominant to dominant. IV–V does the same work by a different route, with the bass stepping up instead of falling, and the two are interchangeable as preparations for V. D minor in first inversion is the more directional of the two, because its bass note is scale degree 4, climbing a step to 5 while the upper voices sink into the leading tone and fifth of the dominant.
Tonic straight to dominant is also available, and common at the start of a phrase. It simply skips the departure.
That is the whole cycle in one line: tonic, predominant, dominant, tonic.
Substitution inside a job
Because function depends on relations rather than identities, chords that share important notes can take over a job. A minor shares scale degree 1 and scale degree 3 with C major, so vi can substitute for I. E minor shares scale degree 3 and scale degree 5, but it also contains scale degree 7, the leading tone, which pulls the music toward the dominant. That is why iii is the weakest of the three tonic substitutes and why it so often appears as a passing chord on the way somewhere else.
On the other side, ii and IV both contain scale degree 4, and ii6 and IV are close to interchangeable in front of V. In minor the same divisions hold, with iv and ii°6 as predominants and VI available in the tonic area. The dominant in minor needs the raised leading tone, which is why A minor’s dominant is E major rather than the E minor triad of the natural scale. Natural minor’s E minor and G major chords behave as modal colour, not as dominant function.
Substituting changes the weight of a phrase without changing its syntax. A progression that reads tonic–predominant–dominant–tonic can be written vi–ii–V–I and still make the same argument.
Where function and chord identity part ways
Inversions do not change what a chord is doing. They change how much it insists. I6 is tonic without being final; V6/5 is dominant with a softer profile than root-position V7.
The cadential six-four is the standard example of function and identity separating. Its notes spell a tonic triad, but its bass note is scale degree 5 and it resolves into the dominant, so nobody hears it as a place to stop. Analysts disagree about whether to call it a tonic chord with two suspensions or part of an expanded dominant. For a player or a writer the practical answer is the same: it belongs to the cadence and it must lean forward.
Adding a seventh to a dominant triad tightens the same mechanism. The seventh is scale degree 4, which has to fall to scale degree 3, and the leading tone has to rise. Two obligations in one chord make the arrival harder to resist than a plain triad can manage. Jazz builds its basic unit, ii7–V7–I, from exactly this behaviour, and the added ninths and thirteenths of later practice change a chord’s colour without promoting it to a different job.
What the model cannot describe
Functional labels are an analysis tool built for tonal music, formulated around 1900 by Hugo Riemann and extended by later theorists. Applied everywhere, they become a category error.
Modal polyphony before roughly 1600 does not fit. There, harmony emerges from independent lines, and a cadence is made by contrary motion and a raised leading tone rather than by a dominant chord doing its duty.
The blues does not fit either. Its I–IV–V carries the same numerals as a tonal progression, but the tonic is a home rather than a goal and the dominant often sounds like a restatement of the tonic. When the third of the tonic chord is unstable by design, dominant function has nothing to attach to.
Modal rock and folk progressions such as i–♭VII–♭VI work by repetition, register and rhythmic weight instead of tension and release. Post-tonal music abandons the premise altogether, and much of the world’s music never had tertian chords to give functions to.
Reading a phrase as functions
Analysis gets easier if you work in three passes. Find the cadence first, because that fixes the key (the article on cadences covers how). Then mark each chord as tonic, predominant or dominant instead of writing numerals. Only then look for the chord that refuses to fit its column; it is usually a chromatic preparation for a local tonic, which is the subject of secondary dominants.
Two habits come out of this. A phrase that will not settle usually has a dominant with no predominant in front of it, so the arrival feels unmotivated. And a phrase that sounds harmonically vague often has its tonic substitutes doing work their weight cannot carry. The circle of fifths tool plays the diatonic chords of any key in sequence, which makes the difference between I, vi and iii audible in a few seconds rather than a few paragraphs.
Questions
What is the difference between a predominant and a dominant chord?
A predominant leaves the tonic and aims at the dominant, usually by carrying scale degree 4. A dominant chord makes the return home sound necessary, because the dominant seventh holds scale degree 4 and the leading tone at the same time, and the tritone between them has to resolve.
Does a ii chord always have to come before V?
No. Any predominant can set up the dominant, including IV or a borrowed chromatic chord, and a phrase can move from I straight to V. The ii-V pair is the commonest route, not a requirement.
