Basso continuo emerged in the seventeenth century as a shorthand notation for keyboardists (typically church organists) who were accompanying a soloist or small ensemble performing a work originally composed for a larger group. For example, if two or three singers were tasked with performing an eight-voice choral work, they could select the most prominent parts to sing, while an organist could cover the rest. Performing five or six lines of contrapuntal choral music could be a significant challenge, so organists needed a way to condense the texture while still preserving the core musical structure to support the vocalists.
The thorough bass emerged as that shortcut. The “thorough” or “continuous” bass is a musical line that includes the lowest note at any given time. Usually, works would not include a single part that could function as this line, so the organist would alternate between voices, as they exchanged registers or took rests. The resulting line, unline the regular “bass” part, was “continuous” (or “thorough”)—hence the name thoroughbass or basso continuo.
A good musician could perform this bass line, and with an eye (or ear) on the vocal parts and with knowledge of how to improvise good voice-leading, that musician could accompany the thoroughbass line with chords that made musical sense. (This is the original meaning of the term accompany—to accompany a bass line with chords. The fact that keyboardists did this in the context of supporting a soloist or small ensemble led to that term later being applied to any situation where a keyboardist accompanied spotlight performers.)
As this technique grew, publishers began publishing thoroughbass reductions of large-ensemble pieces to support smaller groups of musicians. In these publications, figures (numbers above or below the bass line) were included—sometimes only for difficult or non-standard chords, and eventually for most chords, enabling more amateur musicians, as well as students, to make use of the technique. These bass lines with figures became known as “figured bass” lines.
J.S. Bach, Flute Sonata in C Major, ii., BWV 1033. The upper part is played by the flute, the lower part is the basso continuo line, played by a keyboardist who uses the numbers below the staff (figures) to guide the chords played above this bass line.
To this day, harpsichordists performing in Baroque ensembles will often put their left hand to the same “bass” line that the cellos play, and will improvise right-hand chords (with contrapuntally sound embellishment) according to the figures provided with the bass line.
Though most music students are not Baroque keyboard specialists in training, thoroughbass, or basso continuo, can be a valuable tool in the study of harmony and voice-leading. In the study of harmony, a thoroughbass line can play a valuable role as a harmonic reduction of a complex texture, in order to example and understand better the harmonic skeleton underlying a passage. In aural dictation, transcription, and analysis, the bass line and the melody are often the most prominent (and most important) lines in a passage, and knowing how the inner voices tend to relate to the bass line and melody can aid in a number of listening and aural-analysis tasks. In voice-leading, the basso continuo texture affords a straightforward environment in which to make a gradual, staged progression through the intricacies of writing musical lines in a harmonic texture—and to do so without paying significant attention to harmony.