Grade 6 Harmony Lesson A9: Figured Bass Rules for Realization
Realizing a Figured Bass
"Realizing" a figured bass means "making it real" or, filling out the four-part harmony by adding a tenor, alto and soprano part.
Simply building up chords according the figures is not enough though, unfortunately! You need to abide by the rules of harmony as well, and aim to create a musical melody line (in the soprano voice).
In this section, we will exam the rules (unbreakable) and guidelines (break at your own risk!) which you need to memorize, in order to successfully realize a figured bass.
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Voice Leading (Rules and Guidelines)
"Voice leading" refers to the way each separate voice part is constructed, note after note. If you only build chords vertically, by simply looking at the figures, you will end up with a mess horizontally - each voice part will jump around in a crazy, unmusical way.
In this example, each individual chord has been correctly interpreted according to the figured bass, but the resulting individual voice parts are a terrible mess!
Play each voice part through separately, and compare how they sound with the bass line, which IS musical. Better still, try to actually sing these three upper parts - you are likely to find them quite awkward!
With good voice leading, each part will move smoothly to the next note.
- Soprano should move either by step or by a third (guideline)
- Alto and tenor should either repeat the previous note or move by step (guideline)
- Other intervals are allowed if you are stuck, but nothing bigger than a 5th (only the bass part can leap an octave). (guideline)
- No augmented or diminished intervals (if used, there are rules about HOW they are used - best to avoid!)
- Parts should not cross (e.g. the alto part should not go higher than the soprano etc.) (rule)
- The leading note should always be followed by the tonic in the soprano part (rule)
- Don't write an interval of more than an octave between the soprano/alto parts or the alto/tenor parts (an interval of more than an octave is allowed between the tenor and bass parts) (rule)
In the above example, the parts move by unacceptable intervals and the parts sometimes cross. The leading note (B) is not followed by the tonic (C), and the interval between the upper parts often exceeds an octave.
Here's the same bass with improved voice leading. Notice how much easier each part is to sing.
Consecutive Fifths and Octaves (Rule)
As mentioned in the "harmonizing a melody" lesson, consecutive perfect fifths and octaves are not allowed. In the "bad" realization above, the third and fourth chords in the soprano and alto parts contain consecutive octaves - G/G moves to A/A. This is not allowed.
The following are considered to be consecutives and are therefore not allowed:
- perfect 5th followed by another, different perfect fifth in the same two parts
- perfect octave followed by another, different perfect octave in the same two parts
- compound intervals are also counted as consecutive e.g. F-C (perfect 5th) moves to G-D (compound perfect 5th) or vice versa
- consecutives caused by melodic decoration notes, such as passing notes
- consecutives before melodic decoration notes (the decoration cannot "fix" the consecutive)
The following are not considered to be "consecutives":
a. perfect 5th followed by diminished 5th (acceptable)
b. diminished 5th followed by perfect 5th (not acceptable)
c. perfect interval followed by identical perfect interval e.g. C-C followed by C-C (acceptable)
Doubling and Omission (Rules and Guidelines)
When you are writing harmony in four parts you always need to double at least one note of the triad.
There are rules covering each inversion, and you must also make sure that the choice of doubling creates the best voice leading, without creating consecutives.
- In a root position major chord, double the tonic as first choice. If the tonic is not available, double the fifth. Don't double the third (guideline).
- In a root position minor chord, double the tonic as first choice, the fifth as second choice and the third as a last resort (guideline).
- In a first inversion major or minor chord, double the root as first choice, the fifth as second choice and the third as a last resort (guideline).
- In a first inversion diminished chord, always double the bass (which is the third of the triad) (rule).
- In a second inversion chord, always double the bass (which is the fifth of the triad) (rule).
In addition, the following advice should be followed:
- Never double the leading note when it is followed by chord I (rule).
- Always aim to have a chord which contains all three triad notes unless doing so breaks another rule (guideline).
- Only the fifth of the triad can be omitted - all chords need a root and third as a bare minimum (rule).
- If the fifth is omitted, either double the root and the third, or triple the root with a single third. Never triple the third (rule).
The Melody Line
Finally, here are some extra guidelines to help you write a good, musical melody line (soprano part).
- Move by step or by third as much as possible.
- Don't restrict the soprano part to a very narrow range of notes.
- Avoid repeated notes. A note repeated once is fine, but it's enough.
- Aim to make the soprano move in contrary motion (i.e. in the opposite direction to) the bass if you can.
- If both the bass and soprano move by a leap (an interval wider than a 2nd), make sure they move in contrary motion.
- Don't write notes above A (one ledger line)
- Add some melodic decoration (passing notes or auxiliary notes) to liven things up. But watch out - notes of melodic decoration are also affected by the rules of consecutives. In fact, the chords must be free of consecutives both with AND without any melodic decoration (see consecutives, above).