What is basso continuo, and what does it do?
Think about it this way. Basso continuo, which traditionally was made up of two instruments — a bass instrument like the viol, and a keyboard instrument like the harpsichord — was like a rhythm section. The continuo instruments kept the beat and outlined the harmonic changes.
In twentieth-century musical genres like jazz, rock, and reggae, the rhythm section keeps a steady beat which holds the music together. Over this rhythmic and tonal framework, the solo instruments are able to improvise.
For instance, Billie Holiday’s trio keeps a steady rhythmic and harmonic beat, allowing her to come in with her vocal entrances very late (but she always catches up — a hallmark of her style):
The Wailers’ rhythm section does the same thing, allowing Bob Marley to improvise, using his vocals as a solo instrument:
Note the basso continuo (viola da gamba and harpsichord) here, and pay attention to the musical motif they play. It’s called a ground bass.
A famous example of ground bass is Dido’s lament, “When I am laid in earth,” from the opera Dido and Aeneas by Henry Purcell (1689). Pay attention to the bass clef in this piano-vocal transcription:
Another well-known ground bass with which you may be familiar:
This is an example of a score that a continuo player might play from:
It’s not all that different from the lead sheet that a jazz player might use on a gig: