Harmonic function and secondary dominants

Chords in a key have jobs. Some feel like home, some move away and some pull back. Knowing this explains why certain chord swaps work, and it leads to one of the most useful tricks in harmony: a secondary dominant, a dominant seventh chord that leads to a chord other than I.

By the end of this lesson you will be able to:

• Sort the diatonic chords into tonic, subdominant and dominant functions.
• Explain why V7 pulls to I and use this for other chords.
• Play progressions with a secondary dominant in C.

Chords in a key have functions: jobs they do in the progression.

• Tonic (I, vi): home, rest. C and Am in C major.
• Subdominant (IV, ii): moving away from home, preparing. F and Dm.
• Dominant (V, vii°): tension that wants to return home. G and B°.

A typical progression moves tonic, then subdominant, then dominant, then back to tonic: C, F, G, C. Chords with the same function can often be swapped. Am can stand in for C, and Dm for F.

Why V pulls to I. A dominant seventh chord like G7 contains two notes (B and F) that sit a tritone apart. That tension resolves when B moves up to C and F moves down to E, as in the move from G7 to C. This is the strongest resolution in tonal music.

Secondary dominants. The same trick works on any chord, not only I. A dominant seventh chord a fifth above another chord pulls strongly to it. In C major:

• E7 leads to Am (E7 is the V7 of vi).
• A7 leads to Dm (A7 is the V7 of ii).
• D7 leads to G (D7 is the V7 of V).

Each contains one note that is not in the key of C (G♯ in E7, C♯ in A7, F♯ in D7). That note is what gives the pull, and what makes the chord a deliberate colour rather than a mistake.

Progressions to try:

• C E7 Am F G7 C: E7 pulls to Am.
• C A7 Dm G7 C: A7 pulls to Dm.

The pull, in two steps.

1. Play C, Am, F, G7, C at four beats per chord. Listen to how G7 leans into the final C.
2. Now play C, E7, Am, F, G7, C. The E7 leans into Am in exactly the same way.
3. Play the plain version with Em instead of E7. It sounds correct but flat. The G♯ in E7 is what creates the pull.

Try the pattern in G: G, B7, Em, C, D7, G. B7 is the V7 of Em.

Do these in order. Each one is small. Tick the ones you have practised.

• 1.Label the function

  4 min

  1. Write the diatonic chords of C: C, Dm, Em, F, G, Am, B°.
  2. Label each: tonic (C, Am), subdominant (F, Dm), dominant (G, B°). Em is often treated as tonic-like.
  3. Play a progression and call out the function of each chord.

  You have it when

  • You can name the function of each chord in a progression in C.
  I practised this one

• 2.Hear the pull

  4 min · 60 → 80 BPM

  1. Play C, F, G7, C. Notice the strong pull from G7 to C.
  2. Play C, F, G, C with a plain G and compare.
  3. Then play C, Am, F, G7, C.

  You have it when

  • You can hear that G7 pulls to C more strongly than G does.
  I practised this one

• 3.Add a secondary dominant

  5 min · 60 → 80 BPM

  1. Play C, E7, Am, F, G7, C. The E7 is the V7 of Am: it pulls to Am.
  2. Play C, A7, Dm, G7, C. The A7 is the V7 of Dm.
  3. Compare each with the plain versions (C, Em, Am...).

  You have it when

  • You can play both progressions and say which chord each secondary dominant leads to.
  I practised this one

Play at 60 BPM, four beats per chord. These progressions depend on hearing the pull, so do not rush the dominant.

• Treating the chord as 'out of key'. A secondary dominant is a deliberate borrowing. The out-of-key note (G♯ in E7) is what creates the pull.
• Overusing secondary dominants. One or two per phrase is plenty. They are seasoning, not the meal.
• Forgetting the chord it resolves to. A secondary dominant should be followed by the chord it points to: E7 to Am, A7 to Dm.

Many songs use secondary dominants: the A7 before D, or the E7 before Am. Next time you hear a chord that sounds as if it wants to go somewhere, try to name the chord it leads to.

Short version (10 minutes)

Play C, F, G7, C and then C, Am, F, G7, C three times each.

Standard version (about 30 minutes)

Do all three exercises, then play the progression in G using the same pattern (G, B7, Em, C, D7, G).

• Chords have functions: tonic (home), subdominant (away), dominant (pull home).
• V7 to I is the strongest resolution.
• A secondary dominant is the V7 of a chord other than I.
• E7 leads to Am; A7 leads to Dm (in C).

You are ready to move on when you can honestly say:

• I can name the function of each chord in a progression.
• I can explain what a secondary dominant is.
• I can play a progression that uses one.