ATC's, miniature works of art (2½ " by 3½ " or 64 by 89 mm) are created on heavy paper or any stiff backing material (as long as the material is at least as heavy as card stock but thin enough to insert into a standard plastic trading card sleeve). Some cards are now being created that "violate" the thickness requirement. The back of each card usually includes the date, the artist's signature or chop, and any contact information the artist wants to give. It is interesting for collectors to know the card's origin: city, state, & country. Many artists create ATC's with 3-D features: windows, folds, multiple pages, pop-ups, sliding panels, moving parts, hidden messages, and secret treasures. These thicker, more three-dimensional ATC's can be stored in special boxes and containers, rather than the traditional baseball card display pages.