So, this is one of the most common questions after a concert. Right after "Do you guys play Jimi Hendrix?" (No, we don't but we like his music. Thanks for asking, no one has every asked us that!).
But right after that, some audience member will also ask (some out of curiosity, some with a gotcha look), "You played a trio sonata, but you had more than 3 people).
So, in Baroque music, a trio sonata is a an ensemble piece with 3 distinct lines (see the photo).
The first two lines are covered by melody instruments, and this is the easy part to explain. Depending on the composition, they could violins, flutes, oboes or (god forbid) recorders. However, the last line (the Basso Continuo) is generally composed of one chordal instrument (lute, harpsichord or organ) and one single line instrument (cello, gamba, bassoon). This is because they suck and need all the help they can get. The reason for this is that the Basso Continuo line contains information for chords (to improvise upon, like jazz) as well as the written notes for the actual bass. Thus, it is necessary for two people to be playing the Basso Continuo line. In times of financial hardship, it is possible to get by with only one person, however the difference is noticable. In addition, we have written documentation from the performers and composers of the Baroque era that prescribe at LEAST two players on the Basso Continuo.
Hope that was informative!
Next time you see a Baroque ensemble play a trio sonata, you can ask something else! Maybe "Have you heard of that girl to plays violin whilst running through the waves in a bikini?".