There is a saying that generally speaking, an instrument that only plays a single note melody at the same time is a monophonic instrument. Except for the percussion section and harp, all other instruments in the orchestra are basically single-note instruments. Instruments that can play chords at the same time, such as pianos, electronic keyboards, and pipe organs, do not fall into this category. Wind instruments such as flutes, trumpets, French horns, etc. are all single-note instruments. Although stringed instruments can play some double notes or chords, since they play single-note melody lines in most cases, they should also be included in the ranks of what you call single-note instruments.