It was the practice of Baroque composers to write the lowest melodic line - to be played by instruments such as the cello, double bass, and left hand on keyboard instruments - along with some additional figures to indicate to the keyboardist which chords to use. This is called the figured bass:

The instruments that play this bass line and those that realize it harmonically are called the basso continuo.

This practice was later abandoned. However, the figured bass is still used today in the study of harmony.

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