Dozens of different threads run through Baroque music, and tracing them along the century–and–a–half long tapestry is absorbing. The madrigal’s evolution and eventual demise, and the emergence of the concerted madrigal and the cantata in its stead are related examples.
But one phenomenon remained constant: it was in use for some decades before the official beginning of the Baroque, and it continued its influence long afterwards. It has been ascribed various monikers during its career, including the English ‘figured bass’ or ‘thorough–bass’ and the German Generalbass, but is most commonly known by the term ultimately given to it by its Italian inventors: basso continuo.
When treating two or more tones sounding together, a perennial occupation of western European music has been consonance vs. dissonance, formal ways of saying that the resulting sounds are, relatively speaking, pleasant or unpleasant. Various rules spelled out how to handle volatile dissonances and resolve them in acceptable ways, and theories were invented after the fact (in contrast to theories of monody) to explain why these rules were natural and divinely ordered. Fundamental to both theory and practice was the concept we call interval, the distance between two tones based on their relative positions in an ordered succession of pitches, called a scale.
Inherent but hidden in the theories of consonance and dissonance, like the axioms of a mathematical system such as plane geometry or a linear algebra, was a group of notes that always appeared in various aspects but, reduced to lowest terms so to speak, made up the combination 1–3–5 in one guise or another. Nowadays we call it a triad: pre–baroque theorists also recognized it, albeit not necessarily as any kind of fundamental structure. The notes might be spread across several octaves, but an f was essentially the same ‘note’ sung at different heights. Bringing all the notes together as closely as possible always resulted in a triad. In purely pragmatic terms, this was an enormous boon for performers.
Not every church could boast the musical forces of a Rome or a Venice, but its adherents still wanted to celebrate the liturgy, especially on feast days, with appropriate pageantry. The natural substitute for missing voices was to fill in with instruments. The organist was the obvious candidate — he could supply any or all missing voices at once. But reading from separate partbooks was difficult, and writing out all missing parts in partitura (one above the other, i.e. a score) was only marginally easier to use, never mind the tedium of copying out every part. Armed with some practical knowledge, not the least of which was the ubiquity of the triad, the organist could often improvise an adequate accompaniment based on the lowest note.
At first, this was most easily accomplished by writing out only the lowest note at any given time regardless of which voice happened to be singing it, a habit called basso seguente (“following bass”) above which the player could improvise, spreading out the notes of each triad as long as the rules of voice–leading were not too badly broken. Where a 1–3–5 combination wouldn’t work, the player could either deduce the correct notes to play based on an inspection of the highest and lowest voices, or make a notation above or below the basso seguente where appropriate. Thus the 1–3–6 combination — we call it a first–inversion triad — could simply be notated by a figure 6. Similarly, the common suspension of the middle note of the triad before a cadence could be noted simply by a 4 followed by a 3, meaning the succession of tones 1–4–5 to 1–3–5, spread out across two or three octaves.
The advantages of the system were immediately obvious, “easily the most successful system of musical shorthand ever devised” in the words of a famous historian of Baroque music, and its sophistication reached an impressive degree in the hands of skilled players. By the beginning of the seventeenth century, few musicians would think of performing without at least one continuo instrument, whether keyboard, guitar, chittarones or organ. That it wasn’t often mentioned explicitly is testimony to the extent to which it was taken for granted.
Most importantly, the advent of basso continuo had an invigorating, liberating impact on composers exactly at a time when it was most needed. Freed of the mechanics of rigid counterpoint, they could explore new avenues like the expressive setting of texts or new techniques of instrument playing. Spontaneity and inventiveness could take a free hand: the basso continuo was at the ready to support almost anything.