“Der General Bass ist das vollkommste Fundament der Music welcher mit beyden Händen gespielet wird dergestalt das die lincke Hand die vorgeschriebene Noten spielet die rechte aber Con- und Dissonantien darzu greifft damit dieses eine wolklingende Harmonie gebe zur Ehre Gottes und zulässiger Ergötzung des Gemüths und soll wie aller Music, also auch des General-Basses Finis und End-Ursache anders nicht, als nur zu Gottes Ehre und Recreation des Gemüths seyn. Wo dieses nicht in Acht genommen wird, da ists keine eigentliche Music, sondern ein teuflisch Geplerr und Geleyer.“
– Johann Sebastian Bach: Vorschriften und Grundsätze zum vierstimmigen Spielen des General-Bass oder Accompagnement 1738
I’ll confine myself to the beginning of this quote attributed to J.S. Bach: “Der General Bass ist das vollkommste Fundament der Music welcher mit beyden Händen gespielet wird dergestalt das die lincke Hand die vorgeschriebene Noten spielet die rechte aber Con- und Dissonantien darzu greifft damit dieses eine wolklingende Harmonie.”
“The basso continuo, which is performed with both hands in such a way that the left hand plays the prescribed notes while the right hand plays the consonants and dissonants, is the perfect foundation of the music and ensures a harmonious sound.”
Johann Sebastian Bach emphasizes here the fundamental importance for all music from the 17th and 18th century of the thorough bass, the harmonic structure and chord progression.
He knew what he was talking about and was not alone in his claims. Theorists inside and outside the German-speaking world are of the same opinion in different terms throughout the 17th and 18th centuries. In this period, which shows so many stylistic differences per country, region and time, the system of the figured bass was the stable factor. It was not for nothing that musicologists used to call this period ‘Generalbaßzeitalter’.
The example of Friederich Niedt below shows how things could have been implemented.
Friederich Erhard Niedt. Musicalischer Handleitung. Anderer Theil/ Von der Variation des General-Basses, Samt einer Anweisung /Wie man aus einem schlechten General-Baß allerley Sachen als Præludia, Ciaconen, Allemanden, etc. erfinden könne. Die Zweyte Auflage, Hamburg 1721. Das XI. Capitel.
The well-known partimento as we know it from Bernardo Pasquini and Francesco Durante, for example, is also based on this principle.
For the majority of people, listening to and playing music almost automatically focuses their first attention on the upper voice(s). However, it would not be unwise, I think, to always start from the thorough bass (basso continuo) when playing literature from the 17th and 18th century as well.
What Bach states in the above quote is, in my opinion, also applicable to the playing of literature. By playing literature as if you were playing continuo, you get a better grip on the structure of the music, on rhythm and articulation (“do I present my bass here more as a bassoon or rather as a cello?”); your playing gets the tranquillity and reflection and the dance-like character that the music from before 1800 so badly needs. You transcend the ubiquitous agitation that started at the beginning of the 19th century. (First steamship, first train, etc.)
In the case of two voices, the lowest voice is the basso continuo, no matter what the position. This is also the case with three-voice, etc. The two examples below show that even one voice can be seen as a thorough bass – basso continuo.
From: Archibald T. Davidson and Willy Apel. Historical Anthology of Music. London/Cambridge Massachusetts, 1950, 1970.From: Archibald T. Davidson and Willy Apel. Historical Anthology of Music. London/Cambridge Massachusetts, 1950, 1970.
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https://www.bach-cantatas.com/Articles/GBLehre.pdf
Guiseppe A. Pastore: Francesco Durante. Bassi e Fughe. Padova 2003
https://en.wikipedia.org/wiki/Partimento[:]