Basso continuo (It. for “continuous bass” or “thoroughbass”) is essentially a chordal version of first-species counterpoint. However, instead of composing a single line above a cantus firmus, one composes a succession of chords (performed in the right hand) above a bass line (performed in the left hand). Basso continuo writing, also referred to as realizing a figured bass, gives no consideration to melody, only to the use of proper chords and the smoothest voice-leading possible. Thus, basso continuo style is a simple place to begin engaging the “fundamental musical problems” that arise when more than two lines are combined.
In strict keyboard-style writing, there are four voices: the bass line (which is usually a given in basso continuo style), and three upper voices: the melody or soprano, the alto, and the tenor (from highest to lowest). Since all three upper voices must be played by a single hand, they should never span more than an octave.
The melody always has an upward-pointing stem. Alto and tenor share a downward-pointing stem. If the alto and tenor share a note, that note receives a single downward-pointing stem. (See m. 1 of the example below.) If melody and alto share a note, that notehead is double-stemmed. (See m. 4 of the example below.)
When choosing the notes to place in the upper voices above a figured bass, use the bass and figures to determine the pitch classes present in the chord. (When realizing an unfigured bass, you must determine appropriate figures before realizing.) If the chord is a four-note chord, use each chord member once, including the bass (exceptions will be noted later). If a chord has three pitch-classes (a triad, for instance), use each pitch-class once, and “double” one of them according to the following principles:
- If the figure is 6/4, 5/3, or other chord of the fifth, double the bass pitch class.
- If the figure is 6/3 and the bass is a fixed scale degree (do, re, fa, or sol), double the bass pitch class.
- If the figure is 6/3 and the bass is a variable scale degree (mi/me, la/le, or ti/te) or a chromatically altered pitch, double one of the upper voices at the octave or unison.
- Generally, do not double a variable scale degree or a chromatically altered pitch.
In basso continuo style, if the chord is properly voiced (correct pitch classes and correct doublings), two key principles of voice-leading will ensure good counterpoint between the voices most of the time:
- The law of the shortest way (a term coined by composer Arnold Schoenberg): move each voice as little as possible. Prefer repetition to steps, steps to leaps, and one leap at a time to several voices leaping at the same time.
- Move the right hand in contrary or oblique motion to the bass. When the bass leaps by fourth or fifth, though, this rule can be ignored.
In some cases, these rules cannot be followed absolutely (such as when a functional dissonance must be resolved, or when a melody makes it impossible—two cases to be considered later). In all cases, observe the following:
- No parallel fifths or octaves between any pair of voices.
- No contrary fifths or octaves between outer voices.
- Do not approach an octave between the outer voices by similar motion unless the melody moves by step. (All other direct/hidden fifths and octaves are permissible.)