What did the Romans ever do for jazz?
Harmonic analysis of tunes is extremely important to the understanding of jazz theory and hence to the ability to improvise. Roman numerals are used to denote the relationship between the chord and the key, hereafter referred to as RN analysis. The degree of the scale upon which each chord is built (root note of chord) is shown as a roman numeral (ex 1b).
Jazz tunes often modulate temporarily and it is necessary to show these modulations as key centres. Each key centre must be clearly shown before the sequence of chords. (ex 1e: key centres of G and F). In most cases a new key centre is made obvious by a V7-I cadence. (in this case the Gm7/C7-F – chords which are diatonic to the key of F, not G).
Chords are often chromatically altered in jazz so when using the roman numeral system to analyse a chord sequence always remember to add the type and extension of the chord (e.g. m7, maj7, etc). NB: this is different to classical analysis
Tips for RN analysis:
- Differentiate clearly between major and minor keys. (e.g. C = C major, Cm = C minor. Do not use just lower case for minor keys, c is easily confused with C)
- Always include the type of chord and extension.
- Make a note of chords that are not diatonic to the starting key to identify new key centres.
- Look for a tonic chord (maj7 or maj6 in a major key, min maj7, min6 or minor triad in a minor key) preceded by IIm7-V7 to define a key centre.
- Minor 7 chords are very often II chords in a major key. (This is more likely than them being III or VI chords)
- Minor 7 b5 (ø7) chords are often II chords in a minor key.
- Mark the key centre clearly – circle it or use a different colour.
- Bracket IIm7 – V7 together (to highlight the II-V relationship) and draw an arrow from the V chord to the target chord (if it resolves down a 5th) to denote resolution.
In addition to IIm7-V7-Is created through secondary dominants, many tunes are made up of IIm7-V7-Is with different key centres that may appear to be entirely random or may be related logically:
- Misty: key centres in bridge move down a semitone then a major 3rd
- Autumn Leaves: key centres Bb – Gm (major to relative minor)
- Giant Steps: (key centres move up in major 3rds)
Overlapping key centres
As well as the alternative key centres we mentioned earlier – see Alternative Key Centres, it is possible for key centres to overlap. This happens where one or more consecutive chords could be in one of two key centres. In bar 5 of Autumn Leaves the Eb ma7 could either be chord IV of Bb major or chord (b)VI of the next key centre, G minor (based on the harmonic minor or the Aeolian mode). This gives us two possible ways to analyse the first eight bars (ex 9b and 9c below)
The Ebmaj7 is diatonic to both key centres. So although the analysis in ex 9b could be seen as correct, there are various reasons why 9c is better:
- We have not yet established the G minor via a cadence (eg. D7 Gm), so our ears tell us that the Eb is still in the key centre of Bb.
- Chord II(ø7) is an integral part of a IIm7- V7- I cadence.
- The key centres make up two four bar phrases if the G minor key centre starts at bar 5.