What musical notation system uses numbers to indicate harmonic progression below a bass line?

Figured bass is a musical notation system that specifically uses numbers and other symbols to indicate the intervals, or harmonic progression, to be played above a bass line. This system originated in the Baroque era, allowing keyboard players and other instrumentalists to interpret and realize the harmonies implied by the bass line. The figures denote the specific intervals above the bass note, guiding the performer on what chords to play.

In contrast, a chord chart provides a more simplified representation of the chords in a piece of music, often using chord symbols rather than the detailed notation found in figured bass. A lead sheet typically includes the melody, lyrics, and chords, focusing on the basic structure rather than the complexities of harmonic progressions. Score notation refers generally to the written representation of music for various instruments and is not limited to the harmonic context indicated by a bass line. Each of these alternatives serves different purposes in music performance and composition, but figured bass is uniquely designed to convey harmonic implications through a numbered system.