Italian, "Single touch," "single key," or "single subject," tasto solo is a term in group music, indicating where the basso continuo section of the performance should be played without additional harmony, by itself, in contexts where otherwise the continuous bass section of the song would be played by several instruments together.
Typically tasto solo is handled by cello or upright bass, when normally harpsichord or another keyboard instrument would also be featured alongside it to supply harmony to the bassline. Tasto solo is most often used as a way to limit the amount of layer and texture the music has, for a period of time: perhaps the composer wishes to slowly introduce one instrument at a time, and doesn't want the bassline to overwhelm the treble instruments being brought in gradually. Alternatively, the composer may wish to gradually taper off the dynamics of the song, getting quieter and quieter, simpler and simpler, leaving only the bass and a single treble voice together to complete the final phrase of the song. Overall, tasto solo regulates the apparent energy and momentum of the song, as well as its apparent complexity.
Tasto solo is mostly found in compositions from the Baroque era of music, and despite being an Italian term, few Italian composers made use of it, according to German composer Carl Philipp Emanuel Bach, who remarked on its sparing use by Italians.
Tasto solo is not to be confused with sul tasto, which is a specific bowing technique used on violin and viola, unrelated to basso continuo.
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