Final answer:
The middle section of minuet-and-trio form is called a "trio" because it provides contrast and relief from the main minuet theme. It is in ternary form.
Step-by-step explanation:
In minuet-and-trio form, the middle section is called a "trio" because it typically features a different melodic and harmonic material compared to the rest of the piece. This section often provides contrast and relief from the main minuet theme. It is not called a trio because it was originally arranged for three instruments or performed three times. Instead, it is in ternary form, which means it has three distinct sections with the middle section being the "trio".