What is basso continuo
? It's the bottom line of music in most score
s of Baroque music
, or the next-to-bottom line if the bass line is written out independently. Also known as figured bass
, it consists of a normal stave
with notes, rests and stuff, with some numbers (possibly precede
d by sharp
s or flat
s) stuck singly or in columns above various notes.
It isn't the line played by the bass instrument (double bass or violone or violoncello or bass viol -- with the occasional bassoon or bass trombone), although the notes will be identical with the bass line in most circumstances. The notes of the basso continuo (continuo for short) are the lowest notes being played by any instrument at the relevant place within the music.
So, if the basses have a rest and the violas have the lowest line, the continuo will reproduce the notes of the violas. This is where the name comes from -- the continuo continues even when the lowest instruments rest. The reason for this is the importance of the lowest part in Baroque musical theory and practice. (The "bass line" defined by the actual lowest part has a special place in all (good) Classical music as the foundation of the harmony.)
Baroque music, originating in the seconda prattica of Monteverdi, had a very important characteristic: in addition to a number of written-out melody or contrapuntal lines above the bass, there was a chordal accompaniment which was not written out -- rather, it was improvised by one or more continuo players. Although later Baroque composers often created full harmonies with the written-out parts, the earliest Baroque monody was simply a melody and bass, withthe continuo filling in a harmonic background without which the music would sound empty and lack expression. Previously the harmony had been created simply by the coming-together of written-out contrapuntal parts. The new music of Monteverdi might be thought of as a return to the simplicity of the troubadours, although with a greater dramatic range.
Instruments and techniques
Thus, the continuo instrument had to be capable of playing chords so as to provide a pleasing accompaniment to a melody (usually vocal at first). Instruments used for this include the theorbo, archlute, harpsichord and organ. Continuo survived even past the Baroque era in the Italian style operas of Mozart and others: during the secco recitatives, the accompaniment was simply a 'cello and a harpsichord or fortepiano. Other than this, it's a solecism to try to play continuo on a piano.
Usually a (musically) intelligent continuo player could guess from the musical context which chords were required above a given bass note. However, sometimes the composer required something out of the usual grind of I-IV-V-I and II-V-I, hence the numbers and symbols above each note. They consituted an intricate code designed to indicate the exact chord required in the most economical way -- not always very intuitively.
For example a 2 above the note meant to play the second above the bass, the perfect fourth above the bass, and the sixth above the bass, choosing the notes appropriate to the key signature. (Or one could substitute notes an octave higher). So, an inverted seventh chord. However a 9 meant to play the third, perfect fifth and ninth above the bass: the ninth was treated as a dissonance which would resolve to the octave. So, a root position triad with the addition of the dissonant ninth. Two completely different chords, despite the fact that the 2nd and the 9th above the bass are essentially the same note.
The notes of the chord were usually arpeggiated (or spread) upwards, the lowest note being played on the beat with the bass instrument -- thus the rest of the chord would occur after the beat. The speed of arpeggiation varied depending on musical context.
On the rare occasions where the composer did not want any chordal accompaniment, the words tasto solo would be written above the continuo part. This is Italian for "single touch" and means that the bass or lowest part notes should be performed alone or with no other chords than unison or octaves. This might be appropriate in the aria "The People That Walked in Darkness" from Handel's Messiah.
History, part 2
With the increasingly involved harmonies of later Baroque music, the figures sometimes became self-defeatingly thick on the ground. For example the first chorus of Bach's St. Matthew Passion has two or three numbers above almost every note, with sharps and flats liberally applied. At the same time, textbooks were written to fix the range within which the continuo player could use his (or her?) freedom of interpretation and prevent glaring harmonic errors. However, this often resulted in flaming rows between different textbook authors, creating an endless source of dissertation subjects for musicology students and an opportunity for more flaming rows today, as conductors try to work out how to perform the music.
With the advent of the Classical style of outdoor serenades and divertimenti, in particular the string quartet, the bulky and hard-to-tune continuo instruments were discarded in favour of an accompaniment again made up of written-out parts -- the melody instruments did their own accompanying. The new style, despite its inefficient use of the middle parts (dull, chugging second violin and viola parts were all too common) proved easy and popular enough to consign the continuo to musical history, except for a time in the traditional arenas of church music and opera. By the last two decades of the 18th century continuo was dead as a musical idea, and a new era of many-coloured and precisely calibrated accompaniments was well on its way.