Implied Harmony
Although we think of harmony as being chords (usually three or more notes sounding at the same time), even a single line of melody has harmony embedded within it. This is called “implied harmony”, and it is something that is worked out by the listener’s brain, based on their previous experience of listening to music (that is, music already heard in the current tune, as well as music they have heard in their life).
At the most basic level, a tune built on a single repeated note will be interpreted as the tonic, or keynote of the scale. We would not assume that the note is (for example) a dominant, or mediant.
On hearing this tune:
we would not be at all surprised to hear it followed by a chord of B major, as the chord matches the keynote we were subconsciously assuming.
The notes from chords I and V are used to establish the key. Between them, these two chords cover five of the seven notes of the scale:
If we use these steps of the scale in melodies, the ear normally interprets them as belonging to either chord I, or chord V. The dominant note belongs to both chords. Most of the time it is interpreted as chord V, because it is the root of that chord.
The other two notes in the scale belong to chord IV. These three chords (I, V and IV) are called the “primary chords”, because they are the default implied chords.
Here is a melody which implies all three primary chords:
To imply a secondary chord, it is often necessary to use more than one note from the required chord, and to ensure that the root of the chord is used. Using the other chord note(s) as faster note values is helpful,
Implied chords also depend on the harmonic rhythm. Chord changes do not normally happen quickly, and tempo can affect how the chords are inferred. At a moderate tempo or faster, chord changes are more likely to be only every half bar, or every bar.
It is also very important to consider the chord progression (including cadences), conventions of voice leading, and notes which are emphasised because of their position in the bar (on the stronger beats) or relative length. Decoration notes (e.g. passing and auxiliary notes) can also be used.
In this example, which is a slow melody, the ear would have time to perceive a new chord on each beat in bar 1. In bar 2, chord V is implied because the quicker note values here suggest the melody is moving on, rather than halting (which would be chord I), so the E and C are accented passing notes. At the end of bar 2, the implied chord is vi (secondary) not IV (primary), because the note value tells us we have arrived at a cadence. V-vi forms an interrupted cadence, whereas V-IV is not a cadential progression. In bar 3, chord ii is implied rather than vii° because the progression vi-ii forms part of the strong progression of 5ths.
At a faster tempo, the implied harmony may be interpreted differently. The ear does not have time to work out different chords on every beat, and instead, the less emphasised notes are heard as decoration notes.
Here is the same tune at a faster tempo. In bar 1, the D is a passing note, and the F is an auxiliary note. In bar 4, the B is an auxiliary note.
Because there are a lot of factors at play when we interpret harmony from a single line, and we all have different musical experiences, it is often possible that different people will infer the harmony in different ways. If you are writing a single line melody, try to think through the different interpretations that might be possible, and consider which is more likely.