Naming Chords With Roman Numerals
We saw earlier that the degrees of the scale may be named using Roman numerals. This convention is widely used in theory books and we should spend a little time examining the conventions adopted in this form of 'figured bass'. Straight away, it should be pointed out that much is common between Roman numeral figuring and the early form of figured bass set out above.
A chord name should tell us what a chord is; a Roman numeral should tell us what the chord does. In other words, a Roman numeral is contextual (based on key) while a chord name is not.
The triad C - E - G can always be named C (C major) whatever the context or key. But it would be I in the key of C, or IV in the key of G, or V/V in the key of Bb.
The seventh chord B - D - F - Ab can always be named B°7 regardless of the tonal context. But it might function as a vii°7 in the key of C, vii°7/V in the key of F, and a host of other chords in a host of other keys. Of course, depending on context, the same chord might be a passing chord or some other type of nonfunctional sonority, for which a Roman numeral label would be inaccurate and misleading. In that case, the chord name alone, B°7, would be the appropriate and best label.
We discuss chords of the form V/V and vii°7/V in chapter 31.
The quality of the chord is shown by whether the Roman numeral is upper or lower case. Upper case identifies a major or augmented chord (in fact it is identifying the fact that both have a major third) while lower case identifies a minor or diminished chord (which is because both have a minor third). With seventh chords, the case of the Roman numeral is determine by the quality of the triad to which the seventh has been added.
Figured bass, with or without Roman numerals, identifies the notes above the actual bass note by the interval between that note and the actual bass note. Do not make the mistake of working from the chord root, which may not always be the bass note (e.g. with inversions)
Root position triads are left unmarked; the first inversion triad has only a 6 usually, not the more complete 63.
Augmented chords are marked with a plus (+) sign and an upper case Roman numeral, e.g. I+.
Diminished chords are marked with the degree (°) sign and a lower case Roman numeral, e.g. vii°.
Inversions are always indicated when using Roman numeral notation although generally in a more sophisticated way than that we have met earlier when naming inverted triads.
For the seventh chord and its inversions the Roman numeral convention is: root position, V7; first inversion, V65; second inversion, V43; and third inversion, V42.
Roman numerals can be used to indicated non-diatonic chords notes too.
If the diatonic note is lowered or raised by a semitone (half step) a flat or sharp is written in front of the figure. A slash may be used in place of a sharp. If the root of the chord is raised or lowered a sharp or flat will be placed in front of the relevant Roman numeral.
If the chord is non-diatonic, in other words it does not arise from the key of the work, accidentals may not always be necessary. A minor tonic triad in a major key will be shown with i. The flat third is shown by the lower case of the numeral. Similarly, the major triad on the third scale degree (e.g. E major triad in C major) is simply labeled III, here the upper case showing that the third has been sharpened.
When comparing Roman numeral notation with the figured bass set out in an previous section of this lesson, notice that in the absence of Roman numerals, accidentals must be shown. Thus, the minor tonic triad in a major key, i with a Roman numeral, would be marked with a flat sign in figured bass, the flat sign referring to the lowered third in the triad. Occasionally, 'courtesy accidentals' are used to reinforce information already indicated by the case of the Roman numeral under the chord. Other accidentals may be used to show that a note is 'raised' (using a sharp) or 'lowered' (using a flat) when there is actually no sharp or flat in the chord.
So, for example, a II#64 chord which might have no sharp in the chord indicates that a normally occurring flat has been sharpened to a natural.
When there is a change of key it is not unusual to see the chord names in the new key on a second level under the staff. The name of the new key will be clearly marked and also on any subsequent staff. It is not unusual to show the Roman numeral appropriate to both the original and the new key, one above the other during the modulation sequence, or on a particular pivot chord.
Particular chords, some we will meet later, are also specifically indicated by their own letters:
N6 for a Neapolitan sixth
It.6 for an Italian sixth
Fr.6 or F6 for a French sixth
Ger.6 or G6 for a German sixth, and so on.