A period piece is a story set in the days of yesterday (a story set in the future is considered fantasy or science fiction). Road to Perdition, for example, is film noir but also a period piece. The Green Mile (in the fantasy, horror, and suspense genres) is a period piece, too, as is Catch Me If You Can. Unless you’re doing a studio-financed picture with a healthy budget, avoid period pieces, period. They require special art direction, wardrobe, and props dealing with that specific time period.