Thin sheets of metal, or a person's rivals or enemies who make a good contrast with them.
The plural of 'foil,' which comes from Latin 'folium' (leaf) because beaten metal sheets looked like leaves. In Shakespeare's time, 'foil' also meant a contrasting character, like Laertes as Hamlet's foil.
The term 'dramatic foil' is literally borrowed from metalworking—just as shiny foil makes a diamond sparkle more, a character's opposite traits make the protagonist shine brighter in a story.
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