figured bass notation
A Baroque-era shorthand system using numbers below a bass line to indicate the chords to be improvised above it, the foundation of continuo playing.
In Depth
Figured bass (also called thoroughbass or basso continuo notation) was the standard method of notating keyboard and lute accompaniment from roughly 1600 to 1800. The composer wrote out only the bass line, adding Arabic numerals below certain notes to indicate the intervals above them that should be played. A "6" under a note means to play the note a sixth above it; "6/4" means a sixth and a fourth; no figures at all means a root-position triad.
The system required the continuo player (typically harpsichord, organ, or theorbo/lute) to improvise a full harmonic realization in real time from these minimal instructions. This demanded a sophisticated understanding of harmony, voice leading, and style. J.S. Bach was reportedly one of the greatest continuo players of his era. The study of figured bass remains central to music theory education, as it trains students in harmonic thinking, and the tradition of figured bass realization has been revived by the historically informed performance movement.
J.S. Bach could reportedly sight-read a figured bass line and improvise a fully voiced, harmonically rich realization in real time — a skill that even trained modern keyboardists find enormously challenging.