ATCs (or Artist Trading Cards) are small artworks made especially for trading, much like the calling cards of the Victorians or baseball cards. There are only two rules: they must be 2.5" x 3.5" and never sold. Otherwise, any medium or foundation can be used. I've seen them made from paper, metal, fabric, shrink plastic, playing cards and wood. They should always be signed by the artist. They are great for swaps or for trying new techniques and ideas. Here's some I've made in the past.