Piece
Because they are big and have a lot of detail, pieces take a long time to paint and design. This means pieces are more likley to be in places where it is legal to paint them. Illegal pieces are usually in places where there aren't many people, like train yards, tunnels, drains, the top of a building, or walls facing train tracks. Illegal pieces in busy places might be done over more than one night, or by many people at the same time.