Marie Rutkoski said this quote

The beauty of the flute was in its simplicity, in its resemblance to the human voice. It always sounded clear. It sounded alone. The piano, on the other hand, was a network of parts—a ship, with its strings like rigging, its case a hull, its lifted lid a sail. Kestrel always thought that the piano didn't sound like a single instrument but a twinned one, with its low and high halves merging together or pulling apart.