Why were trio sonatas often played by four instruments?

Although the term "trio sonata" implies a composition for three parts, it was standard practice for the genre to be performed by four instruments.12

The music was written in three distinct parts: two high melody parts (typically played by violins) and one bass line. To perform the work, the bass line was played by a low melody instrument, such as a cello or bassoon, while simultaneously being supported by a keyboard instrument, such as a harpsichord or organ.13

This keyboard player provided a basso continuo—a system of improvised accompaniment where the performer realized the harmonic implications of the written bass line. Because the bass line required both a melodic reinforcement and a harmonic realization, the total ensemble required four performers to cover the three written parts.132